Hmm
Halfords.
Can't say much about Boardman or Carrera bikes, as I don't own any of them.
My previous bike was from Halfords. Setup was a bit on the naff side, and after a few days I had an issue with the gears and due to this the chain came off and snapped, as it wrapped itself up between cassette and spokes!
The after care from this was equally shocking, basically my branch didn't really care, and were far too busy to replace my chain with a new one! Although much complaining later I did get what I rightly deserved, and some new spokes fitted, due to some damage.
Such an experience has put me off Halfords for a bike. Purchased the new one in a LBS.
I still use Halfords for the odd thing, mainly car shampoo etc...
But I'm sure there are people out there that have had good experiences.