I don't know of any difference between the GS and the GSUK - only that the UK version will be set to not output more than the statutory 100mW allowed by Ofcom, and maybe one or two other settings.
No, v5 isn't and won't ever be supported due to it running on VxWorks - but try getting PC World or any other bugger to tell you which OS it's running!
It can easily be checked when received anyway - and if it is v5, it's just a case of getting it back to them. They can't stop that happening within seven days of receipt.
WUSB54GS is a bog-standard adaptor, with the added advantage of it being speedbooster compatible.
At the end of the day, if you're after a WRT54GS, you can't be this; let's just hope it's not v5 though!