PLEASE would you all just phone up HMRC. the phone call is not expensive and their customer service reps are not cops or dictators that will chew your head off.
The chances of calling HMRC and speaking to anyone who has even the faintest idea what Bitcoin is (least of all, how it should be taxed) would be pretty close to zero.
The grey area is what happens if you do not cash out the Bitcoin but rather use any gain to directly purchase goods and services (I know that's pretty difficult right now in the UK but it won't be for long). If I bought £100 of BTC back in January 2013, by 2014 I would have approx £10,000 worth of buying power in BTC...according to what HMRC have already published I could spend these directly and incur no personal tax liability.
There's a short discussion here on AccountingWeb as somebody posed a similar question:
http://www.accountingweb.co.uk/anyanswers/question/tax-bitcoin-gains-hands-individual