Account for bitcoin transactions in the same way you would track how much money you've made from them. Need to track £ price in and out, to work out final profit in £, then declare it under self assessment.
I'm starting to think most people won't need to even declare it under self assessment. This year's self assessment asks two questions:
- Did you dispose of chargeable assets worth more than £42,400?
- Are your taxable gains more than £10,600?
If we assume mined coins that are sold for £ represent a capital gain of the entire value, there is therefore no need to declare them unless the total received in a year is over £10,600. At current market rates, you'd need to be mining around 0.37 BTC per day to hit that limit. Even if you exceed that limit, provided you only sell £10,600 of them per year, you still wouldn't need to provide details of any of it on your self assessment, because you'd answer 'no' to the above two questions.