£50 seems steep for a front when merlin are selling the set for £78
Sets are always effectively cheaper than single wheels, I'd guess it boils down to sale and supply. They sell far more sets than single wheels, so it's more cost effective for the manufacturer.
Buy the set and sell the rear, sorted
[EDIT]
EBAY, it's down for collection only, but if that's not convinient then you may be able to educate the seller in the use of Hermes/TNT/DHL/other courier of choice
Or,
THESE might be worth keeping an eye on.