Yup, it’s a living and inventive language with new uses all the time.
How do you know what it means? You just do. In the same way that you would know a regional argot in another area.
There are areas of Britain that find each other mutually incomprehensible - eg london and the North East.
Anyway it’s half twelve here and it’s time for my Giorgios (giorgio Armani - Sarnie)
For those of you on the other side of the pond … sarnie = sandwich. 
Unless I missed it, no one posted my favorites, which I learned from my Yorkshire husband:
US “I don’t care” or “no, thanks” = UK “I’m not bothered” (This one led to some awkward moments when Rick first moved here. For example, a waitress would ask what kind of salad dressing he wanted, and since he doesn’t eat salad dressing, Rick would say “I’m not bothered.” In the US, that just doesn’t parse. If it means anything, it means something like “Salad dressing does not annoy me.”
US “I don’t want to,” “I don’t feel like it” or “I can’t be bothered” = UK “I can’t be arsed” (I love this one, and use it as often as I can, as in “I should do the dishes, but I can’t be arsed.”)
This drives me particularly nuts, because other languages in Europe (in my case, German and Hungarian) use the half+hour construction to mean “half of the next hour.” So “Halb neun Uhr” or “fel kilenc ora” (“Half nine” in German and Hungarian respectively) means 8:30, and my brain always stutters when I hear that construction.
Also on recent trips to the UK I have noticed that Budweiser is quite popular along with other lagers such as Stella Artois and Fosters (yuk)
Re: Rhyming Slang - would it be possible for its convolutions to circumnavigate the sphere of meaning?
Scene: NATO wargames operations center/centre a few years ago
British Officer: “…and as we aproach these coordinates we should begin to encounter our rushing opponents.”
American Officer: “Understood. Ohm uh, not to be pedantic, but, as you know they’re not all technically ‘Russian,’ what with all the ethnicites and Eastern Block and all.”
British Officer: "Oh, I didn’t say ‘Rus-ee-an,’ I said ‘rush-ing,’ which is a play on words for “charging,’ short for ‘Charging Elephant.’ Which comes from the elephant’s ivory, which is a pun on Ivory Soap, which is from “Soapy,” which is rhyming slang for ‘Soviets.’ But don’t worry, by next week it’ll come full circle and ‘Soviet’ will be the slang term for ‘Soviet.’”
American Officer looks into corners for hidden camera, thinking there’s been a Candid Camera-meets-Monty Python show launched.
us-kiss uk-snog
What is an off-licence?
It’s a liquor store. So named because they are licensed to sell alcoholic drinks to be consumed off the premises .
If you look above the entrance door of most pubs you will see written
Joe Blogs licensed to sell alcoholic beverages to be consumed on and off the premises
so in fact most pubs are also off-licences.
I can remember when many pubs had a separate off-licence section. This had its own door and counter. Some used to be called *Jug and Bottle * because you could take your own bottle or jug and have it filled from the beer barrel . The pub had to use a calibrated receptacle to do this to satisfy weights and measures regulations.
Not sure about this one. A snog is a particular kind of kiss.
Anheuser-Busch has done a good job of positioning Bud as a relatively high-end beer in the U.K., which strikes this U.S. transplant as one of the greatest marketing feats of all time.
Especially when you compare it to the “real” Bud. I know which I prefer.
raises hand
I say “quarter after” fairly often.
Me too. The only way I ever heard references to what time it is, in Canada, was “quarter to” and “quarter after”, along with “eight thirty”. When I came down here, I had to ask what “quarter of” meant, on a few occasions. It still doesn’t suggest to me anything about what time it is…
“It’s a quarter of eight.” “What, you mean two?”
It’s only drunk by 20 year old Darrens. No one else will touch it.
I usually say “quarter of” and “quarter past” on a regular basis.
US cars have tires and carburetors.
UK cars have tyres and carburetters.
A UK raincoat only strains the rain a bit: to keep the rain out you need an oil-skin.
UK sod> the guy you’re arguing with, aka an asshole or dummy.
Irish sod> coal substitute made from peat moss.
US sod> a carpet of live farmed grass which is then planted on bare soil for an instant lawn.
That’s arsehole, arsehole!
Enola Straight, in Ireland it’s not usually “sod” it’s “turf”.
Turf can also refer to the ground in general or horse-racing in particular. It’s a bit confusing, but “I’ll put some turf on the fire” is acceptable if you want to chuck some peat on it.
I just found a wonderful (and wonderfully-appropriate 
) site: Britain for Americans
Part of the reason for this is that the vehicle licence in the UK (that is, the object that asserts the vehicle is permitted by law to be used on public roads) is a paper disc - the Vehicle Licence Disc (AKA ‘Tax Disc’), usually displayed in the front windscreen.
In order to get a licence disc, you have to present:
-Either the registration document or the renewal reminder (essentially to establish that it is actually your vehicle)
-A valid certificate of insurance
-The MOT test certificate (to establish that it is nominally roadworthy)
So if you’re driving uninsured and/or driving an untested (i.e. possibly/probably dangerous) vehicle, or one you don’t legally own, you can’t ‘tax it’ (get a licence disc) and you can (in theory) be more easily spotted by the rozzers. Of course it only needs renewing every six or twelve months, so it isn’t a perfect or even moderately speedy system.