Check out http://www.cockeyed.com/incredible/solardish/dish01.shtml.
Rob Cockerham's been there, done that (although with small mirrors rather than Mylar on a 12-foot diameter dish). He also has a number of videos on YouTube showing various items subjected to his "light sharpener."

For seeing the sights in London I heartily recommend London Walks (requires Javascript). They feature several trips which go all across London and have many for particular locations like the British Museum (which has 13 acres of floor space, so it's easy to get lost) which can optimize your time by hitting the highlights for general touristy stuff (leaving you time to go back for further perusal of those or looking for off-the-beaten-track items of personal interest). Do take advantage of some of their day-long trips to places outside of London such as those to Salisbury and Stonehenge, or to Canterbury and Leeds Castle, or to Bath--especially if you plan on staying in London for the entire time. Visiting only London and saying you've been to the UK would be like visiting New York City and saying you've been to the USA. If he is giving tours at your time in London, I suggest taking in the Jack the Ripper walk with Don Rumbelow (who literally wrote the book on Jack the Ripper).

Someone recommended getting the Oyster Card, but you might check to see if the London TravelCard may be better for you. The TravelCards are only available to people visiting from outside the UK and can be cheaper than getting an Oyster Card. Here is a side-by-side comparison. Note that the TravelCard can also be purchased through the AAA.

Check with your bank and your credit card companies for their foriegn exchange fees and use the best card for your purchases and walkabout cash. Please check NOW--you don't want to find out that your cards are unusable overseas when you get there. You might avoid the convenience kisok ATMs in favor of those at banks for obvious reasons.

Above all else, remember: you will NOT see everything!. You can literally spend years seeing the sights of London alone much less those of the UK, so don't try to take it in all at once by limiting your visits to, say, the British Museum solely to an afternoon. Take your time and enjoy your time away from work, and if you miss something this time you can always catch that particular sight the next time you visit.