One of the bigger B&Q stores with a Trade Point counter. The little ones don't carry as much stock.
They will also help plan it out for you.
If money is not an issue then you can try Magnet and Southern. Much more expensive and no difference in quality, but sometimes have better designs.
Best place to buy doors would be either howdens joinery there nation wide or wicks granite is a specialist material search local granite installers hope this helps
The only problem with B&Q, Howdens, Wickes etc is that they double drill the doors.
Search the internet for a smaller company near you that does door replacements, they will be able to supply a large range of doors at better quality, and drilled the correct side.
Kind Regards