... in response to Euro Power Converter for, posted by tim360z on Aug 29, 2002I got mine at Fry's. ($50 for the whole kit).
But your laptop probably doesn't need a power converter... just a plug (which is a LOT cheaper.)
Check your power supply. It will probably give you a range of accepted power voltage. You should be fine. Most can hande from 110 to 250.
BTW, many places in London have receptacels that accept both kinds of plugs. You may not need anything. (The admiral's club, for example, accepted both...)
Also, the airport probably has what you need, anyway. Gatwick's international duty free plaza has stores that sell the plugs and converters. You can probably wait until you get there, see the plug you need, and then simply go buy one. I don't think it will cost you any more than getting it here.
FWIW.