Hello. I’m curious to learn more about what you’re looking for. What type of guild - casual, hardcore, etc; whether you want to focus on raids, M+ or fishing; what goals you have - do you want to do Mythic raiding?
I’m also unsure as to the reason why you’re specifically looking for a British populated UK based guild. And not sure how you’d define “UK based” 
Servers, as far as I’m aware, are in France, so with the best will the world, EU guilds will be based there 
You also haven’t mentioned if you’re happy to jump realms.
Joking/questioning aside … I’m guild leader of a Guild on the Connected realms of Lightbringer/Mazrigos. I’ve been part of that guild since the start in 2007. It was set up by myself and another Brit who has since left the game and I’ve been running it for the majority of that time by myself, or, latterly, with a few officers. We currently have three officers, two of whom are from the UK (one in the middle, one at the bottom - what can I say, I’m geographically challenged )
We are an INCLUSIVE guild … we don’t exclude people based on where they live or what experience they have in-game, so we have people from everywhere. We do have a lot of Brits, but we also have South Africans, one or two from the US, Poland, Holland, Norway and no doubt a bunch of other places (again, rubbish at geography )
We do have an age restriction - ages range from 20ish up to erm, lets say almost retirement age Most people fall somewhere in the middle.
We raid twice a week, currently both Normal and Heroic Sanctum. We have groups that organise M+, Torghast and occasionally a PvP night.
We are a casual guild, and are happy for people to learn how raids or M+ work for the first time, with us, in a friendly calm environment.
We also welcome those who wants to perhaps try out some M+ with us, without joining the guild, to our discord channel, where often M+ are organised (selecting the M+ role helps if you do that, so you get pinged when they’re organising, because it’s often at that time): https://discord.gg/gQ2uBGe
Please also see [H] Dark Wolves - Normal/Heroic raiding, Mythic+ & Socials Guild for more information.