I bought these for a London trip. Came in extremely handy.

Samsung candy bar phone sealed and new, SIM unlocked $30 240V AC Adapter, will accept US Voltage, just need a physical adapter or maybe another samsung charger would work.

Samsung candy bar phone used a few times $27

These phones will not work in most of the US. I heard D.C. has these bands though for travelers.

SIM Card for UK $5 (will require an unlocked phone with 900/1800 GSM bands) Will require a 5UK pound top up card.
" " " " $10 - has a couple bucks balance on it x 2
Oyster Travel Cards x 2 with 1.10 UK pounds on them $10 each

Have some UK pounds left too. Nice to have cash, tube travel cards, and a UK cell # before you go. You DO NOT want to use your phone, the roaming fees are ridiculous.

Right now, the SIMs I bought have free phone to phone calling until 3/31/12 which was nice!