Agree with all that Mas said regarding food. Makes your eyes water when you see the prices. Could not believe how cheap most things were. Being in the food industry found I had to keep going in to different shops to check out prices and make sure I wasn't dreaming.
Agree on petrol and transport also, however a couple of tips.
If doing London you need to have an "oyster card" for the tube and I think it works on the buses as well although we did not use it. It reduces your individual trip prices dramatically (when paying for each ticket) and is easy to use. If landing at Heathrow it can be purchased there for immediate use on the piccadilly line - not the airport express. Tricks attached to oyster card - there is a 5 pound deposit, which is refundable when you finish your travels and cash the card in. They tell you that you can get the refund of the deposit and any credit on the card by going to any station, which is true. However they don't tell you that the maximum they will refund is 10 pounds in cash. They will however send you a cheque if the amount is over 10 pounds which of course is totally useless. So be careful how much credit you put on the card, particularly near the end of your travels.
If using the train to travel through the country book early and you will save heaps. There are many booking sites (I use d Virgin Trains as there was no booking fee. I did London Norwich first class for around 15 ponds whilst a friend did same trip standard class paid on the day of travel and it was around 60 pounds from memory.
If you need any other hints on train travel - google "The man in seat 61"
