What I can certainly see is a fragmented 2012 fundraiser, with certain donors staying with WMUK and others switching to WMF because that's where the default landing page now points them to. Lots of returning donors will be very very confused either because they can't gift-aid their donations, or because two "Wikipedia organisations" are asking for their money at the same time.
The split has always been a problem, ever since WMUK Version 1.0 in my own mind....
Gordo