You could get the 4-shaft and never run out of patterns to weave.
But yes, if you want to do doubleweave (double width, or a tube, and so on) in patterns other than tabby / plainweave, then you would need 8 shafts. And there are, of course, many more gazillions of patterns available to you with the extra shafts.
If you can afford the £50, I would most definitely get the 8-shaft. If you've got competing uses for the £50, though, then you might want to think about it a bit harder.
It's also worth mentioning that the Ashford stand and treadles a) only offer direct tie-up (one treadle per shaft), and b) only offer treadles for 4 shafts. So if you'd been planning to use the loom foot-driven, the extra 4 shafts would still need to be hand-manipluated. (I suspect this doesn't apply in this case, but thought I'd add it for any subsequent readers coming across this thread.)