Kitting out your home office (and avoiding injury)

I, like many others have been working from home the past year. I am lucky to have a summer house in the garden to work from. I have been getting by with an old second hand desk and chair but recently been experiencing some shoulder pain. Some research into why this might be the case revealed I probably need a chair with better support.

I have therefore decided to invest in some new kit. I am willing to invest a fair few quid seeing as it is something I use for hours per day. However preliminary research has revealed that the best executive chairs cost almost 2 grand.

Does anyone know of any office equipment outlets South East London / kent area? I don't mind if its second hand but would like the ability to test out the kit.

Thanks (bracing myself for a bunch of chair jokes :D)