[Motel75]

Don't bother buying off eBay. Just go shopping when you get there. Have a look at Carphone Warehouse (which is everywhere) or one of its competitors, or just go to a Virgin store (also everywhere). You can always call your friends in the US and tell them your number; it's 20p a minute from Virgin. (Be sure your American friends have an "international calling plan" or an international calling card; my sister did not, and using her home phone to check my German voicemail and say hi to friends cost just under $4.50 a minute instead of 8 cents. This is apparently common practice with several phone companies, in this case Verizon.)

If you have a GSM phone that supports BOTH 900 and 1800 MHz, you're in luck, you don't need to buy one. However, several networks now require dual-band (900/1800) phones, although depending on their legacy frequencies, you might be able to get it to work anyway if you only have one of these.

However, if your phone is not quad-band, my recommendation would be to look for a phone with SIM, as these can be very cheap, sometimes GBP 20 or so. On the whole, I'd recommend Virgin, because it has reasonable prices (first 5 minutes of every day is 15p, thereafter 5p), international roaming, and you can keep the account going forever just by placing a single call once a year, in case you're planning a return trip. OTOH, if you've got a phone that works (or can get one cheaply), are just planning a single trip, and are primarily calling abroad, go for the mobileworld card from Carphone Warehouse. 15p national calls and low international rates, though no way to keep the account from dying if you're outside the country for 6 months as there's no international roaming.

If you don't know your way around London, you can always go to Tottenham Court Road on the Underground, which is at one end of Oxford Street, and you have Virgin Megastore, Carphone Warehouse, and several other stores withing a short distance.