Okay, ex-barman and bar manager chipping in.
The important thing to remember about tipping in the UK is that we do not have a tipping culture. in 99% of jobs, you never expect a tip. You expect to get paid a good wage to do the job, and expect to be able to charge exactly what that service/item should cost straight up. We don’t like “hidden costs” as, well, there’s something fundamentally unfair about them (and yeah, we do have a national thing about fair play). A tip should represent an appreciation for doing something either particularly well, or above the generally accepted standard.
Basically a tip is something you earn, not something you expect. (I could talk for hours about the differing experience of pubs/bars in the UK and US on this subject and others, but that’s a whole other thread!)
Admittedly this culture is starting to change and get “americanised” in some places, but these are mainly your upmarket, trendy, wannabe posh places - gastro-pubs and bottle-only bars being the prime guilty parties. The practice of putting your bill/check on a little tray and thereby “hinting” that a tip would be nice is becoming increasingly common and is (i think) downright rude. As a general rule i never tip in those places unless the service has been particularly good or its a complex cocktail order. Not least because you can normally guarantee that in such places you’re paying an unfairly high price for the drink in the first place.
Outside of those places, however, if you genuinely feel that someone deserves a tip, then i’d say that the three different methods below represent those that are most acceptable in the UK.
- "…and have one for yourself"
This is perfectly acceptable and will normally result in one of the following replies:
-
“Oh cheers” (followed by the barman/barmaid pouring themselves a drink). This tends to happen in the quieter, more local pubs, ones who aren’t owned by chains and those that have a small number of regular staff. Basically anywhere that has a more relaxed atmosphere and/or code of practice.
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"Thanks - i’ll take it later (followed by the barman/barmaid putting the money for the drink in a tip jar). This was the standard practice in my bar - basically because we had a lot of temporary staff pass through (who you couldn’t necessarily trust to be sensible about drinking on duty) and it could often get busy.
It was up to the staff whether they took the money at the end of the night or had an actual drink, and there was a pretty even split between those who did and didn’t. Personally if there were any of the regulars i liked in, or if the person who left the tip seemed like a nice bloke (or cute girl 
) then i’d take the drink and sit with them (if i wasn’t intruding). Otherwise i’d take the cash and convert it into either a chinese takeaway or possibly a beer with some mates on the way home.
Sometimes those tips are pooled, sometimes not, it’ll depend on the place. In my case they weren’t - i used to make pretty good tip money because i was a good barman, as were some others at the place. We knew our regulars, knew who’d been waiting longest, were good talkers when necessary and generally kept good bar. The temporary staff were often lazy or downright rude to both us and customers (it was just another couple of hours work to them, nothing special) so there was no way we were sharing our tip money with them.
- “Nah, you’re alright there. Thanks for the offer though.” You’ll actually hear this reply more often than you think. Don’t forget that the majority of bar staff aren’t expecting a tip, so can actually be reluctant to take one in whatever form it takes. This is because most barstaff work on the basis that if someone is offering to buy you a drink and you don’t think you’ve earnt it, they’re either:
a) drunk as a skunk
b) hitting on you (more common for the girls obviously!)
Both of which can end in tears. Some places actually have a flat “no tips” policy because of this.
- "…and don’t worry about the change mate"
If you’re paying with a note, and the total cost comes to somewhere relatively close to the total, then this is perfectly acceptable. By close to the total, you can generally use a “50 pence per note size” rule as a rough guide - so 50p per fiver, a quid* on an ayrton, one-fifty or maybe a couple of nuggets on a pony etc.
This is closer to an actual moneyed tip, but gives the barman/barmaid the flexibility to do whatever that pubs general practice is with that money as well as not feel any particular obligation to you for taking the tip. That means it can either go in the tip jar (if thats their policy) or the till (if its a small independent place or has a no tips policy), either way there is a benefit for the barman/barmaid beyond the financial as they now don’t have to worry about spending the time getting you change, which is always appreciated - particularly if its a busy night.
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"…and you can wack the change in the tin." – or – quietly put the change in the charity box
Most pubs will always have some kind of collection going on. This will either be for a big charity or, in smaller communities or more local pubs, often a collection for some local need - this may be community based (a collection to keep a church or community centre going maybe) or for an individual (a local who has fallen on hard times, or has died and needs a good send off etc). If you feel the need to leave a tip, but aren’t sure about how it would be taken, then this can be a good way of doing it (as long as you do it without it seeming like you are trying to impress people by giving to charity!), particularly if that money is going to support a local cause. Word of warning though, if it is a local thing, its always best to ask what its for first!
As Gorilla pointed out, in some places (often in poorer areas) people simply don’t have the cash to tip so it can be seen as a bit posh. Also, some people (as i’ve already touched on) can actually resent or feel guilty about getting a tip if they don’t feel they’ve earnt it - think about it this way, if you are paid to do a job then you should generally do it to the best of your ability if you take pride in your work, not because you expect some kind of extra financial reward. I used to feel this way sometimes with people who tried to tip me after just buying a basic drink or having a quick joke with them - stuff i considered to be part of my standard job. Done right, this charity method is always a good way of avoiding that and everyone gets good karma (the charity, the pub and you the punter).
The landlady of one of my old locals, for example, was heavily involved in sending terminally ill kids to disney land. There was no point buying her a drink, as she’d always point out to people who tried that technically she owned all the drinks already! She had a collection tin at the end of the bar though, so most people would (if they wanted to leave a tip or show appreciation if she’d given them a free drink) just tell her to put the random change from their rounds in the tin. After all, two pints came to about 4.70, so 30p change from a fiver wasn’t exactly going to rock your pocket. Over the year though, that would all accumulate and help send a couple of kids on a holiday they’d never have otherwise had, and she kept a noticeboard and scrapbook of pictures and letters from all the kids that had benefitted over the years. It was nice because everyone, whether they were a trendy businessman or the bloke who ran the fruit and veg stall on the market, got to contribute to doing something for the community, and i’ve seen more than one scary-looking thug type go all misty eyed over the letters that pub used to get.
*i wanted to type the number/symbol combination for one pound, but after spending the last five minutes trying to find the pound (sterling) sign on this damn American keyboard i’m gonna give up, and you’re going to get slang from now on in. So don’t complain about it - it’s your own bloody fault.