Wasn't there also an issue with WMUK processing payments, in that they couldn't pass 100% of the donations on to the WMF but had to retain some or most of the funds in order to remain independent under British law? That seems like a good reason why the WMF might not want WMUK to process payments, and the byzantine ruleset they'd have to abide by to have a regime of chapter-based processing is a very good reason to avoid that model in general.