Classic question. The ones that are bought out by large (usually American) foreign companies almost always get worse over time.
Bad examples: Cadbury - Mondelez Hotel Chocolate - Mars
Good examples (afaik): Montezuma Booja-Booja Gnaw
Any others?
Audrey’s Chocolates - based in Brighton
Monty Bojangles - London based, British Owners
Mrs Tilly’s - Larbert based, Scottish Family Owners
Mackies - Aberdeenshire based, Scottish Family Owners
Buchanan’s Confectionery - Greenock based, owned by Golden Casket
I’ve bought Kernow Chocolate for a few Christmas gifts before - it’s really good: https://www.kernowchocolate.co.uk/
Whitakers - recently bought by Bramble Food Group from Leicester. However, a Whitaker is still in charge and their factory remains in Yorkshire.
Demarquette Fine Chocolates - still family run in London.
Cwtsh Chocolate - still run by its founder in South Wales.
If we are in the spirit of buying European, then Thornton’s is at least Italian.
King Charles III: every British citizen is allowed to own chocolate. It’s a law. Praise me.