As others have said made in the UK is few and far between although there are some.
My recommendation would be to look into Stanton bikes.
They have 3 bikes in their current line up…
Swithback…full in hardcore…lots ride them all day but they really are capable of some abuse. Think DH HT/Enduro/Trail ridding. (27.5″ Or 27.5″+)
Slackline…trail HT…all day bike, not as knarley as Switchback but still very strong and very capable. Think Enduro/trail/XC. (27.5″ or 27.5″+)
Sherpa…trail/XC HT…all day bike, very capable 29er. Think Enduro/Trail/XC. (29″ or
27.5″+)
All three models come in Steel & Ti. Also all frames go through the full testing on one frame so they’re very strong products. I’ve only ridden the Slackline & Switchback which are sorted, apparently the Sherpa is to, I’ve very very tempted to trade in my Slackline for a Sherpa.
There’s a great ‘Stanton Owners’ group on FB, if you want to see/find out more.