Do you have a picture of the sort of pocket clip you are looking for?
Knifekits in the US has a good selection, but like everywhere in the US, the options available for shipping very small parts to the UK is limited and are absurdly expensive. $28.80 to ship a $3 pocket clip! I bet that it would work out cheaper to ship to a person in the US and have if forwarded, or to have it picked up by someone travelling there.
It might also be possible to make one by hand by bending a suitable thickness of titanium strip. Probably more time and effort involved than it is worth though.