As a Londoner, I will give you some advice:
Buy your food from the supermarkets when you are on the go, tesco, sainsburuy's, coops, morrisons, or waitrose. cheaper than individual restaurants and shops in C.London. If, you are in greater London, buy them from there.
Check on line
to check if there are any engineering works or strikesÂ

it means some lines might be suspended.
http://journeyplanner.tfl.gov.uk/user/X ... anguage=enBy the way, I hope you have Oyster card now, if not, buy one, it is cheaper for transport!
It worth it also to check online for prices or discounts before going anywhere.
Like most places, people are nice, but careful with money and any other equipments you are carrying, put them always where you can see keep an eye on them.
As for places to visit: Covent gardens is my favourite, they have cheap street shows thereÂ

Also, this might be useful:
http://www.timeout.com/london/Greater London is good too, it is not typical for tourists, but if you get board from C.London, you may give it a go, try Wimbledon for example.
If you need any help for any reason, you may PM me.Â
if I think of something else, I will post it
Â