As a Londoner, here are my 2 cents (or 2 p’s):
1. Oyster cards have a deposit charge so remember to return them when you leave, or else you’d pay more than you should.
2. transportaion is really expensive – so just take it easy. a sinlge trip on the tube costs 4£, and a day travel card 7£. if you’re planning a whole day of looking around, get the daily travelcard.
3. Try pub food. Even the most small, dark and seemingly uninviting pub usually has great food to offer, and it is much, much better than going to Mcdonalds or Subway.
4. The bus tours actually suck. they really do. if you do insist on going on one – make sure you pick one with a live guide and NOT a recording. Remember that the weather will most likely prevent you from sitting upstairs on the open part of the double decker – and you lose a great deal of the experience.
5. Apologize all the time. This is a big part of being British, so try to become a part of the great “sorry, excuse me, pardon me” fest.
Have fun!