I've ordered from World Import before. They have the adapters at $1.99. Experience has been that they're at least £5 in the UK so it's a huge savings to order a bunch here if you need them.

Rather than carry a bunch of plug adapters, what I now do to reduce my load is use one plug and a Outlets To Go strip. Not strictly designed for 220v but I haven't had problems and have used one heavily (for laptop, cellphone, table lamp, etc). Just be doubly sure that whatever you plug in is rated for 220v as it doesn't convert voltages!!!!

Regarding getting the proper UK power cord for laptops, etc... strictly speaking it's safer as the plugs can short/fail/etc. The most prevalent plugs I see also aren't fused and thus violate UK electrical code. That said, 13amps is a *crapload* of current for electronics and many devices would've melted down before blowing that large a fuse. Moral of the story; unless it's for permanent use, the adapters are fine.