I’m thinking ubiquitous companies like the big chains: e.g. Sainsbury's, Tesco, M&S, Waitrose, et cetera.
Or perhaps would it be better for me to ask on Travel Stack Exchange instead?
There's a huge difference between USA/Israel vs. UK. In the USA, the OU are on top of their game. Most stores in the USA, even 1000 miles away from a Jewish hub, have plenty of OU goods on their shelves, with hechsherim. In the UK, you need to download/buy/look up the KLBD Kosher guide, which lists things that are kosher. The vast majority of these things don't have any hechsher on the packet, so you are relying only on the guide.
Regarding meat, I've pretty much only seen that sold in Orthodox kosher stores/butchers, and rarely if ever in any big stores like Sainsbury's. Best head to London, Manchester or Gateshead to get hold of some!
Check out the kosher kingdom in Golders Green https://kosherkingdom.co.uk/