I am based on the borders of london and surrey a place called cheam, this area there are a hell of a lot of cyclists since 2012 olympics, we are close to box hillBuy on spec and condition, not brand. You'll likely pay more for the bigger brands (like Specialized) for a less good bike. Find some options near you (don't buy blind on eBay) that are in your size and budget, post the links and let people comment on whether they are a good option
You'll have to do some research and legwork yourself...especially as people don't know where you are based.
The Voodoo brand for Halfords is popular for entry level MTBs eg https://www.ebay.co.uk/itm/284384333934?hash=item4236a0906e:g:EC4AAOSw7hRg~RwK