I’d second not visiting Tussaud’s, the London Dungeon, Leicester Square or Harrods.
Tussaud’s is kinda fun, but really expensive in a city where most tourist attractions are free or way cheaper than Tussaud’s.
The Dungeon simply isn’t in the slightest bit scary and, if you’re there in high season, you’ll be in the queue longer than in the attraction (you’re guided around rather than wandering freely); if you like that kind of thing, the London Bridge Experience/London Tombs is much better and right next door.
Leicester Square simply doesn’t have much to offer. Trafalgar Square is good to go to these days, though - they have lots of events, usually free, lots of space to sit and chill out, and the National Portrait Gallery is right on its edge. The fourth plinth (the only one in the square not occupied by a statue) is currently host to actual people, and that can be entertaining sometimes.
Harrod’s does have some decent food in the food hall, but the rest of it is overcrowded and snobby - groups have to enter via sperate doors (even if that means sending non-English-speaking children out onto the busy street by themselves), you have to remove your rucksack, the security guards are all on a power trip, and the Dodidi memorial is hilariously tasteless.
Oh, I’m pretty certain most English women would melt at being called Ma’am in a rich Southern accent.
Don’t get the tube from Charing Cross to Embankment. Actually, don’t get the tube around the West End at all (though you might need to arrive there by it). Walking distances are miniscule and buses are clearly marked and hard to get lost on.
Don’t get a London Pass. You wouldn’t be able to visit enough of the attractions in the time limit without either losing money or seeing so little that it’s not worth it. They’re also cheeky bastards; they have a London Pass which incluces travel, but they charge more than normal Oystercards. They even charge for children, who travel free. (Children do have to pay on some suburban overground train lines, but the maximum fare is two quid per day).
An Oystercard is a must, btw - you can order them online before you come here, but they’re also easy to get at the first station you use.
Don’t leave tips on the table in a pub. I’ve only seen one tourist do this, a very nice American Doper, actually, but the money was more likely to be stolen by the next people to take the table than ever getting to the bar staff. If you want to tip the bar staff, say ‘one for yourself’ and they’ll add 3 or 4 quid to your order. However, some pubs (like the Wetherspoons chain) don’t permit bar staff to take tips.
When tipping in restaurants, give the tip by cash if you can be arsed, not by card. Many restaurants keep the tips rather than passing them on to the staff who earnt them.
And please don’t act like an inconsiderate tourist on the pavements or on tube platforms in Central London. You’re probably less likely to do this than the average tourist, since you are actually asking for advice, but most of the pavements are very narrow and it’s really bloody annoying to get stuck behind a group of people standing stock still looking around, while you’re trying to get to work. It’s easy enough to do your gazing after moving to the side of the pavement or somewhere else less obstructive.
I think this might be a city vs. country thing more than a tourist thing, though, since I’ve seen visitors from smaller places in the UK do the same thing; they just don’t realise how packed the streets can be, so don’t adjust their behaviour.
BTW, if you go to the Natural History Museum (which is excellent), the queues in high season are ridiculous. But that’s at the main entrance. There’s another entrance round the corner which leads to the Earth section of the NHM, and there are never any crowds there. That part’s worth seeing for itself, but it also leads to the main museum with the dinosaurs and so on. In high season, going that way could save you a good hour or two of queueing.