Just speculation, but if the Brits have seen the chink in out armour, they may attempt to build or just endorse a HMS -UK exchange. Presumably followed by a heavy crack down on (read: regulate into oblivion) all other exchanges, perhaps blocking or restricting any international exchanges.
ID verification to enter unique accounts and hence the potential to claim tax of wealth leaving the system.
-- you bought 200BTC last year sir ... where are they now? because you've not been paying your capital gains tax.
Obviously this would be highly negative in terms of liberty, but it would be a massive boost if not the final straw in terms of legitimacy
'Times 03/Jan/2015 Chancellor on brink of adopting Bitcoin as official currency for UK banks ...'