Ok I've done my research on this recently.
Do you want and overlap or shiplap?
I've just bought a shiplap 6x4 from Tesco direct for just under £170 including delivery.
Focus do a 6x4 overlap for £99 (instore only) but I don't know how quickly you'd get it as it says it's a 28 working day delivery lead time.
Wickes do an overlap (instore only) for £125.
Wilkinsons plus do a 6x4 overlap for £140 plus £19.95 delivery.
Or an 8x6 for £225 plus delivery.