Yeah they are technically trade only, although some branches will deal with the public. They were set up as a subsidiary of VAG in the UK in order to compete with the aftermarket like EuroCarParts and GSF, and so the parts counters in the dealerships became internal or public facing and all the trade moved to TPS.
Most of the time I just call them with a shopping list of part numbers, which saves them having to look things up like they do for all-makes garages who call and ask for part X for model Y. I'm not aware of any online ordering facility if they have such a thing, but chances are you'd need a trade account anyway.
If you go in to your local branch knowing exactly what you want, and with part numbers if possible you may find they're happy enough to deal with you, but they may send you to the dealership instead. Tbh I don't know how TPS's retail prices compare with the dealerships - they may be the same, in which case its all irrelevant!
