I have the Haworth Fern chair below and love it. It is expensive, but worth it in my opinion. You don’t want to skimp on something you sit in all day- and I could sit in this chair all day without pain.

My only other piece of advice is to schedule breaks to stand and stretch. I had a standing desk for a while and I did not care for it. I never got used to standing and working- maybe I am just not coordinated enough. But taking standing breaks helped with my back pain and improved my focus.

Good luck!

https://store.haworth.com/products/fern-office-chair?sku=SFTE-U0-7S01A6,4H_FT,MB_SL,TR_7,PM_P

The UK uses miles. They mostly use the metric system, but there are a few cases where they use the imperial system (like distance)