Try them all, except Action Bikes on London Road and Halfords. My best advice to you is take your time over the decision, try out as many bikes as you can before making a purchase. What price bracket are you thinking?
Nothing too extravagant pricewise, just something to 'do a job'... I'm a complete novice when it comes to bikes, I don't even know what a decent make would be so all help appreciated!
I may get a bit salesman on you here now. I used to work in Evans Cycles in Brighton.
First thing I would say is, do you want a mountain bike because you think you might want to ride off road, or do you know you will definitely be doing off road riding?
If it is the second option then get mountain bike, otherwise get yourself a hybrid bike.
If you do want to go off road, you will need to spend £500 minimum, ideally more, to get a bike that will do the job. In terms of makes do not get hung up on who made the bike, most of them come out of the same six or seven far eastern factories any way. I advise you to visit all the bike shops in Brighton, try every single bike you can. Have a go on a few expensive ones too, so you can feel the difference and get a better idea of what you like and don't like. You will be able to tell which bike feels right to you after trying lots out.
The two best shops are Sydney Street Cycles or Rayment Cycles in New England Road.
Sorry, but they just are..........
Come to Berkshire and try Pedal On in Tadley , Banjo Cycles in Newbury, or Mountain High in Pangbourne. I'd rate them all just as highly 'cos I've bought my kit from them. Pedal On and Mountain High have great web shops too. Banjo Cycle really requires a personal visit.
Banjo Cycles fixed up my bike better than new after the Police recovered it after being nicked, it had to have new wheels and brakes, and they made it even better than before.
I'm just trying to answer the question asked by the OP, in my opinion I'd go to a Marin or Specialized stockist. It's all down to the staff in the shop at the end of the day, and neither of the two I mentioned would try and give you the hard sell ( which I detest ).
It's a shame the Boardman range gets tarred with that brush, as they are very well, if not the best, speced bikes within each price bracket.Firstly don't buy from Halfords. Know exactly which bike you want to buy ( research first ) then get best price off the web as it will save you money. If you want to splash the cash, get one built to your specification