DO NOT USE B&Q!
They sub out to a large firm who then subs out to the person who bids the lowest, which is usually a one man band. The fitter will work as fast as is humanly possible, cutting all kinds of corners so they can get onto the next job. The money B&Q's subcontractor saves by subbing out to the lowest bidder is then basically used to sort the numerous problems you will have with the installation.
Had a £4.5K B&Q bathroom install over a year ago, basically discovered the above for myself. Last week we had the floor relaid after the grout started falling apart and one of the (expensive) tiles cracked due to the fitter using too thin a plywood layer beneath. We also had a leak through the ceiling, and had to have a hole cut to access the pipes.
There are numerous other issues that we've had that I don't have the energy to write about now, but trust me when I say that it is good advice that I am giving you by saying you should avoid them like the plague. I have met other people who have had worse experiences than myself too, so mine is not an isolated case!