Whats the difference between the 2? is there only a very slight difference in the size?
The unf is imperial (inches) and has a finer thread so capable of higher torque rating (tighter)
the M10 is metric (10 milimetres) and slightly bigger with a coarser thread pitch
Both SHOULD be ok as the difference in diameter is around 0.5mm
Forget B&Q and go to an Industrial Fastener supplier, I use a company called Anixter in Chesterfield(Derbyshire) there will be someone simlar near you 8)
Steve