If it's not too thick or too big, Wickes sell 6.5x1220x2440mm Birch ply for £27.89. Dunno if that is plus the dreaded VAT.
In my (very) limited experience, ordinary thin (3mm) ply is OK for outlines, but if you try to do anything delicate, like the portrait I posted, bits can easily break off. Don't ask me how I know, or how much it ticks you off when it happens after about 2 hours. Grrrr. Having said that, someone with more experience may not have that trouble, but as Gary says, there may be voids in the cheap stuff, and in fact, that what caused mine to break.