[trying to offset bad karma of sarcastic raleigh chopper suggestion]
Your 1st proper mountain bike yes?
You will defo be able to source bikes cheaper off tinternet, and nearly but not quite all £500ish hardtails will be a good first buy. See above for suggestions.
If you’re on the slippery slope to mtb obsession, inevitably you will want to upgrade everything, and by then you’ll maybe want something more specific based on your experience of what works for the riding you do. I’d say don’t worry too much about the brand of bike you ride, just get something at about £500 and enjoy riding it.
A good bike shop who will make sure you get the right size, give you good advice, put you in touch with other riders (v important!), service your bike etc etc. That’s worth paying a premium for when you set out methinks, and will start you off on the right foot. They won’t be half so eager to help if you trot in with a bike bought cheap on tinternet.
I’d spend more time sussing out the shops in your area and worry less about a specific type of bike to be honest.
[/trying to offset bad karma of sarcastic raleigh chopper suggestion]