I would suggest a local builders yard or wood yard.
B&Q are expensive when it comes to wood and I have found wood is cheaper
at
the wood yard,thats locally by me though.

I found the opposite. B&Q were about 2/3rds the price of the local timber
yard for floorboards. Also, the timber yard would only sell random lengths
and stated flatly that no-one would sell me floorboards in regular lengths.
The conservatory I needed them for has a metal framed construction that
required no joins, and B&Q had fixed lengths only 15cm longer than I needed.
The decision was a no-brainer, really.

Christian.