http://blogs.wsj.com/moneybeat/2014/09/11/heres-what-the-bank-of-england-thinks-of-bitcoin/The bank estimates some 20,000 people (of a nation of 60 million at the moment) in the U.K. hold bitcoins. And daily transactions number about 300 or so.
As a proportion of the U.K. economy thats obviously tiny: In all the £60 million of bitcoins circulating in the U.K. represent a tiny 0.003% of British broad money balances. All of which means that bitcoin currently represents no financial stability risk to Britain.