Go to http://pc-mobile.net/gps.htm for GPS cables. If they don't have what you need, no one will. I think they are based in Hong Kong but I have ordered several cables from them for various Palms and they have always had fast delivery here in the US so I asssume that the UK shouldn't be a problem either. Their prices are also very reasonable.
The part number is PDCT-GE. This part number includes both a 6 inch RS232 converter cable to match the T-series Clie, as well as a 6ft eTrex adapter cable - and both of these can be used separately. For instance, the eTrex cable could be used with a laptop as well, or the Clie adapter can be used with a cell phone.