Okay, I know a lot of you were a bit worried about cost of living in the UK right now. As far as food is concerned, the grocers you'll probably be using would be Sainsbury's or Tesco. Both stores have online shopping, so if you want to get a sense of how much you'll pay to eat per week you can do it right at those sites. I'll also give you another fantastic site called xe.com This website is a currency converter online and works very nicely.
If you play around with the grocery websites and xe.com you'll get a solid sense of how much everything will cost.
If you like Brit Pop, Lily Allen actually has a song where she mentions Tesco. It's kind of the Wegman's of the UK.
If you're planning on taking your Mac Book Pro to London you'll need a voltage converter. The Apple Store sells one. It's 39 bucks, but you might be able to find it cheaper elsewhere. EDIT: Reading some reviews on the Apple Store website it seems that for our purposes you can get a similar thing for much cheaper at an electronics store. Also remember that the London Center has computers there, so you don't have to take your computer to London.
Since the UK uses a different electrical system than the US you'll also need voltage converters for any appliance you take to the UK, like a curling iron, hair dryer, etc...
Along with the converter, you'll also need a plug adapter. This seems like a lot of work, right? Well, if you go to Wal-Mart they usually have full packs of adapters, etc... in their Travel aisle.
So since I just pimped Wal-Mart, I'll give you a link to K-Mart.