Several people have mentioned my recommendation of A C Computer Warehouse, which I'm happy to continue to endorse.
However, when I recently needed to source a new computer for a friend (who, like Woodelf, only uses a PC for office-based computing, rather than things like gaming), A C Computer Warehouse didn't have anything in stock which really seemed to suit my/his requirements. So I looked elsewhere and found a charity seller (with excellent feedback) on eBay, flogging decent refurbished computers for really low prices. My friend was really pleased the computer I ordered for him (and, as I set it up for him, I can confirm that it's an excellent product).
So I'd happily buy from the same seller again. The computer I'd go for from that seller's current offerings is probably the HP ProDesk 400 G3, with a 6th generation i5 processor, 8 GB of RAM and a 128 GB solid state drive. The price is £109.99, including carriage:
https://www.ebay.co.uk/itm/324507868429?hash=item4b8e2d1d0d:g:nW8AAOSw3jBggEYd
(If you're interested in that model, Woodelf, I can order it for you in the same way as I did for your other recent eBay purchase. You'd need to get someone to set everything up for you of course; I spent around three hours configuring my friend's new computer from that seller).