As a currency of El Salvador, BTC is now exempt from CGT in the UK.

S.35 of the Finance Act (2012) (https://www.legislation.gov.uk/ukpga/2012/14/section/35) amended the Taxation of Chargeable Gains Act (1992) so that gains made on withdrawals from foreign bank accounts are not liable to capital gains tax with effect from 6 April 2012. In the process, the requirement (s.252 of the TCGA (1992)) https://www.legislation.gov.uk/ukpga/1992/12/section/252 that the foreign bank account(s) must represent currency acquired by the holder for personal expenditure outside the United Kingdom, was also removed.

Edit 1: To avoid pointless disagreements here are the definitions I'm using: a money is a store of value (doesn't have to be a commodity, sorry Mises/Schiff, perfectly acceptable to be volatile); unit of account (can be used to denominate prices even if not in wide use) and medium of exchange (has a clearing system, in BTC's case, built-in).

A currency is a money that is 'current'; in other words is acceptable for a wide range of use cases. I would say that any money that must be accepted as legal tender (the widest use case) deserves to be called currency even if it's used by a minority of citizens. A country can have more than one currency and more than one legal tender.

Legal tender is narrowly defined as what courts will accept in discharge of a debt. Citizens don't have to accept legal tender BUT Section 7 of the El Salvadorean bill specifically says that creditors must accept BTC, which makes it much closer to a currency than a legal curiosity.

It's quite clear that it is intended for BTC to be a currency and impossible to argue why BTC is not a currency in El Salvador although the dollar is, since there are no practical differences in operation.

HMRC's position stands on one leg - which is that BTC is not a money or currency. Simply repeating that statement over and over doesn't mean it's true or it won't ever change.