Theoretically you could end up in a lot of trouble making if not an RFD and OP with his licensing. You cant even perform work with Op standing over you.
Any part is deemed a firearm even a screw which could so happen to be off the shelf at a diy shop! The Laws an arse, but.
Theres a big difference to making if you don't know what it is. An RFD can sub out parts manufacture to an engineering company no questions asked. I dare say that RFD could sub out each individual part pressure bearing or not, the idea is the RFD is seen to be bonifide and regularly checked by that constabulary. Theres nothing stopping the OP from making the said part himself.
The part is dead easy usually done by apprentices 10 mins tops hardened and tempered but ideally a degree of fitting will be needed. That pin slot needs to be at the right angle and width along with other criteria there could be extraction probs/breakages.
Knowing most of the dealers down that way I would say they aren't interested for the sake of £35 the going rate fitted. Hence high speed lathe for one, wasn't in Knightbridge or just outside was it? Looks very familiar but I am useless with names, best of luck.