Okay Americans, I know this will sound dumb, yet it may happen. I found that nearly everything in London is double the averag price as in the states. The reason I mention this is because it is easy, for me at least, to forget that because when you look at prices, they appear written the same.
Example: A tin of biscuits for a souvenier will be priced 9.95GBP. My mind quickly thinks, "ah, $10.00, that's reasonable". Then my brain says, "hold on, that really costs $20.00, NOT so reasonable". Or, a soda from a vending machine. 1 pound, "oh, average". Ooops, $2.00 "yikes".
What I am saying is, you need to concentrate when buying incidentals so you done over spend your budget. Of course, you are on holiday so hopefully it won't matter.