I just called HMRC about this (13th April 2013), to at least tie down capital gains vs income tax
"buying and selling Bitcoins using your own money definitely comes under capital gains tax"
HMRC telephone advisor 13th April 2013
The advisor explained that it is treated like buying and selling a foreign currency.
Next time ask them to point you in the direction of the law and governance covering such this (seriously).
Also, how is it treated as buying/selling a foreign currency when it is not recognised as a currency.. Again, I think it should be treated this way. But until it says it in the law it is not .. no matter what me or anyone says on the telephone. Want points of law, links, show it.
Check my post above to see/understand for yourself how Capital Gains Tax simply doesn't cover it. Unless there's chunks of Capital Gains Tax law that I'm missing, and if I am, please point me in the direction, because HMRC can't seem to.
Think I'll be contacting HMRC myself over the next week or so to try to get more clarification..