IS the tax 20% capital tax charegable on bitcoin profit and any amount exempted .
Seems like you'd only have to pay capital gains tax on it when you convert it back to fiat.
I've found another source which explains this more in dept, though you should really talk to an accountant if you want to be completely sure.
If the bitcoins have been purchased HMRC will regard any increase in value as being liable to capital gains tax. Tax will only crystallise when the bitcoins are converted into another currency, be it sterling or dollars or even another cryptocurrency. Capital gains tax is currently charges at ten per cent or 20 per cent (or a mixture ) depending on the level of the taxpayer’s other income.
http://www.thisismoney.co.uk/money/experts/article-5019947/Do-pay-tax-sell-bitcoin-big-profit.html