Knee high leather boots - a tall order. I do know they make wide calf boots but knee high… not sure. You will find mid-calf styles.
I’m in the US so I don’t know which brands to recommend for UK other than HOTTER (which I have many shoes), then try CLARKS. Go to online websites for your major dept store chains and start cruising. Good luck… You may just find what you’re looking for.
I never go shopping without my small retractable 6 ft measuring tape. I’m always measuring for length, inseams, organizers, lamps, flower pots, sauce pan lids, pictures - it could be anything! So valuable to have!
It’s always in my purse.
USA Gundy