Hi Maxwell
Danfoss in Denmark provide the "
Tantaline" process which I think is some kind of CVD process - I assume this is the current process being used on your fasteners. The link to website above is interesting, though take with a pinch of salt as obviously commercial bias/marketing may be rife. The only other process I think may be applicable to your application will be electroplating from molten salts. Other processes like thermal spray, cladding, PVD etc I think will not be suitable due to thick coatings, line of sight processing...
Unfortunately, no UK based company pops out. GlenB's suggestion is a good starting point.