If you're a company selling Bitcoins HMRC state that Bitcoin falls into the VAT bracket of a 'credit voucher' (https://bitcointalk.org/index.php?topic=153332.0*). If I remember correctly VAT is only payable if you; a) turn-over 79,000 (2013-2014) GBP per annum, b) are registered as a company/sole-trader and c) are selling the Bitcoins.

For the individual, they treat it like an asset (like a painting by an expensive artist) - therefore you wouldn't pay any tax unless you liquidated to fiat. In this instance it would fall under capital gains rules, so - if I recall correctly - you get the first ~9000 GBP of any gains tax free (2013-2014) and then any further gains (from Bitcoins, Shares, Currency Trading, etc etc) are subject to capital gains rate of 18-28% (see: https://www.gov.uk/capital-gains-tax/overview)

* There is a better thread on the forum where some user actually linked a screenshot of the written response from HMRC