Re: Oh, the irony
You are wrong. Currency gains are subject to CGT.
From the HMRC manual:
"When foreign currency is held in a bank account and the account has a credit balance, the account is an asset for the purposes of capital gains tax.
For capital gains purposes all gains and losses must be computed in sterling (see CG78310) so on the disposal of all or part of this asset a capital gain or loss may arise."