mrv3000 | England info.

So say I was going to England. Not that I have any definite plans to, but I'd really like to sometime in the next year.

What's a good time of the year to go? What's a bad time?
Since it's not like I can spend a month (probably more like a week), I'd probably be limited to the London area. What are some OMGMUSTSEE/DO things? I'm thinking some of you guys would know better things than a tour book. (One day I really do need to spend at the British Museum otherwise I will most likely 'splode.) Recommend anything - attractions, restaurants, pubs, events...
Although time would probably be limited, I wouldn't mind hearing about things that would be worth a train ride/overnight trip to other parts of the country.
Any other helpful hints? Like...OMG. What if I run into the Queen at a McDonald's?? Do I curtsy? Offer a fry? Ask her to sign my right boob? No, wait. England. Left boob.

ETA: I really want to make a crop circle while I'm there.