Of course, as many posts have touched on, we'd need to agree on what "still and truly British" means. It could be some combination of
- does it matter where are they headquartered?
- does it matter where their products are (final-)assembled?
- does it matter whether most of their components are British?
- does it matter whether they are privately owned (and if so, the nationality of their majority owners) or a PLC (shareholders may be transient/anywhere)?
- if a division of something bigger, does that automatically make them "un-British"? Or if they are an autonomous division of something overseas-HQ'd does that maybe count?
It's either complicated or, no offence to the OP, a meaningless concept.
Most hifi equipment relies on a huge number of components which are imported, whether DAC chips, speaker drivers, cases, or whatever. Let me list a few "British" favourites, all preceded by AFAIK and followed by a question mark:
ProAc: Scandinavian drivers assembled in Britain into British made cabinets by an independent British-owned company
ATC: British drivers assembled in Britain into British made cabinets by an independent British-owned company
Sugden: feels quintessentially British (owned, assembled) but no idea where the components come from
Music First Audio: British-wound transformers assembled in Britain
dCS: not sure about components or cases but... British owned, HQ'd, assembling in Britain
Arcam: formerly British in most senses, then acquired by US company (for IMHO their product hay day) and now owned by Harman-Kardon