hi Alex welcome to the wonderful world of mountain bike riding.
there are many options open to you, but a good place to start is…how much do you want to spend?
£0-300 gets you an old(ish) bike with no suspension but maybe half decent components on from yesteryear.
£300-600 could score you a nice bike with front suspension forks and disc brakes front and rear. look for specialized/giant/marin. good forks might be rockshox recons or fox vanillas for example
£600+ you might want to search out an entry level 'full suspension' bike, with disc brakes and better things hung off it i.e xt gearing(105) or better wheels(mavic/hope), look out for trek/spesh/orange(if you are really lucky!)
£1k+ getting a bit more bling/committed to the sport, shiny shiny kit xtr(dura-ace) rockshox pikes/fox talas forks.
this is not an exhaustive guide and i am sure other people will have opinions that differ from mine.
have fun and remember that 'its all about the downs!'