Despite what you might think, it’s not simply a case of looking on eBay or asking watchmakers to find parts.
I’ve had a large number of vintage Omega serviced by the guys I listed above and all are great, however, on multiple occasions I’ve been advised by the same guys that a certain part is required that they just can’t get and the best course of action is to speak to STS.
After several years I use a ‘best athlete’ approach which includes using all of the watchmakers I’ve listed above depending on the requirements of the watch in question. If after years of buying vintage watches in the UK STS aren’t a company you use, then good for you. But to suggest there’s no need for the parts they can access as all other watchmakers can get hold of the same parts is inaccurate.