I guess that would also apply to zaps too. As the GBP value is so small and you get a certain amount of capital gains tax allowance, it shouldn't be an issue. Unless you are zapping a huge amount 😀⚡
If you buy anything with btc you are essentially selling it. The UK hmrc requires you to record the GBP price at that point and use that for capital gains calculations.
Not sure how it works in other regions but I would imagine it is similar.