You can't pay tax on 100 Bitcoins until it becomes a taxable event, which would be the gain you make when you change them into GBP.
If you take your income (income tax already paid) and then gamble it, there is no further tax to pay whether you win or lose.
I think the problem is how you class the initial acquisition of the bitcoins.
I think that's likely to be the taxable event, probably as Income Tax (although this would depend if you mined on a Pool, individually etc). When you sell them / give them to someone else, that's probably a second taxable event, this time as Capital Gains (increase in value between the first and the second event).
There after, yes, if you gamble them with someone who is licensed then any gain or loss is probably exempt of tax.
This really needs an accountants opinion. Preferably one who is also prepared to sign off your tax return.
Costa Rica, where a number of online gambling companies are based, doesn't require a gambling licence. Satoshi Dice is based in Costa Rica.
Yes.
