Andrius
That's a nice budget to be starting with, it should get you a decent hardtail. How much research have you done, and do you have any fixed ideas about what you want to do with it?
Do you want new, or would you be prepared top buy secondhand and risk that some components may/will need upgrading/changing?
What sort of riding do you have in mind? A lot of people overestimate the sort of riding they're going to do, and think they need all sorts of bling kit just to ride bike paths and woodland trails. Others underestimate and end up with too little bike for the sort of thing they want to do, and end up trying to ride trail centre black routes on a rigid towpath bike.
I'll assume you want to ride rough bridleways and will be tempted to go and try the gnarly stuff at trail centres.
The very best you'll get new would be a Canyon Grand Canyon 6.0. They're a bit over your budget at £768, but the level of equipment is second to none for the money.
At the moment your budget would buy you a Cube Acid, or a discounted Cube Ltd. Decent forks brakes and wheels, some decent gears but low-budget chainsets. Very capable, very pretty, but be aware that the Germans love to ride forest trails and fire roads. Their bikes can be nervy for British trails and off road routes.
Genesis make some nice alloy MTBs, and the Giant Talons are meant to be designed with British riding in mind. Check their offerings.
The big guys like Specialized and Trek will sell you a bike for £700, but in my experience they will be lower end components on a more expensive frame . That's fine if you envisage upgrading in the future, but you'll end up with a cheap fork and wheelset at that sort of budget.
If you can get to a
Halfords and see past the prejudice, Carrera and Voodoo hardtails all hit the spot at your budget. Similarly
Decathlon will sell you some awesome kit for your dollar.