No, you have to pay taxes on the 100 btc you made. The fact that you then gamble the money has no bearing on the fact you have to pay tax on the 100 btc. If your line of thinking was true, then I could gamble all my income every year in a 99.99% winning game and not have to pay any taxes.
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.
See CGT at
http://www.legislation.gov.uk/ukpga/1992/12/section/51/enacted"It is hereby declared that winnings from betting, including pool betting, or lotteries or games with prizes are not chargeable gains, and no chargeable gain or allowable loss shall accrue on the disposal of rights to winnings obtained by participating in any pool betting or lottery or game with prizes."
The Bitcoins are clearly winnings from betting, and so are expressly exempted.