Linn and Naim all seem to do quite well. They seem to have maintained loyal followings since the early 1980s and some of their classic products are still in demand . I've never been a paid-up member of the flat earth society [1] myself, but I don't doubt the quality. Quad used to have this reputation, but they have been acquired by a conglomerate and I believe now manufacture outside the UK. They able seemed to go through a phase of frequently updating products.
Michell Engineering is more of a boutique operation - they only make turntables and tonearms. They are still going in North London: they refurbished my 1983 turntable a few years ago.
Going beyond music. In photography I would say Leica. In terms of how long the products last: I'm still using an 1934 Leitz lens on a current Leica digital camera (so 83 years). If I needed the lens refurbished for some reason I can still send it back to Wetzlar. I'm not sure if it is significant, but Leica has a significant following of collectors. I do wonder if the high prices they are willing to pay for limited editions of products (think gold plated finishes, custom engraving) helps keep the brand profitable.
[1] For Bruce in the US. Somewhere in the depths of time, a British reviewer or journalist coined this phrase to refer to the adulation for the (admittedly very good) combination of Linn turntable and Naim amplificaiton.