I'm not too sure about the availability in the UK, but I found this site on Google that might be able to help:http://www.linuxwireless.co.uk/usb
Also, Christophe's link is a valuable resource :)
I am also looking for a USB wireless adapter myself to run on a MacBook with gNewSense. Currently I have the Alfa AWUS036H-V5 that uses the
RTL8187L chipset which works out of the box with gNewSense, but I'm looking primarily for a simple USB plug instead of a whole setup with antenna
and cable. Being a retailer of Linksys wireless series I have tried the WUSB54GC
on a Acer Asper TimeLine 5810 with gNewSense without any luck.
gNewSense listed it as a unknown network controller, though I see the WUSB54G is recommended on FSF' list of hardware.