True, except it's not only that people don't bother to report holiday cash transactions - there are formal exemptions from capital gains tax in the UK for (i) cash which is for personal use while travelling outside the UK and (ii) (since 2012) for all foreign currency bank accounts. In terms of what most people do, it's only if you hold or use the currency for investment purposes that you need to worry.

HMRC initially tried to treat Bitcoin in the same way as gift vouchers, not currencies, which meant that different rules applied (particularly in relation to VAT). However they now apparently treat is as a currency which in theory means the above exemptions are open to people to claim.

There are issues with this though. Firstly, in the legislation "bank account" is not explicitly defined so it's unclear exactly which forms of wallet might qualify for that exemption, if any. Secondly, the exemption for physical currency doesn't include personal use of a foreign currency within the UK.

So it's still pretty unclear whether you'll be subject to CGT or not.