I had the same problem with an ebay dealer.  The
UK doesn't do International Postal Money Orders, 
so that makes sending them money difficult.  
I couln't find a bank that would do an 
international bank draft in pound sterling for
a reasonable fee, so I ended up sending cash
which I hated to do and I would recommend only
as a last resort.  If you do send cash, send it
registered or certified or something and request
a return receipt.  I did some research, and 
it seems you can send money Western Union to the
UK.  They also have a thing where you can send
money over the internet kinda like paypal, but
you both have to sign up for it and I don't know
if the other person would be willing to do it.
If you go to www.westernunion.com, you can 
go through a whole little thing where it will
tell you how much it will cost you to send an 
amount of money to the UK.  I think it's about 
10 bucks.  I hope this is helpful!

Jen