This is not UK based, but in general, you absolutely do not want to be forced to pay taxes on the hypothetical gain at the end of the tax year.
Assume you bought 100 bitcoins at 0,10€. If they raise to 10k€ at the end of your tax year, and you would have to report it as taxable gains, you will have to pay taxes on 1 million €.
Unless you have that money available, you probably would have to sell some of your bitcoins at 10k€ just to pay for the taxes. And then you would own less bitcoins when they are at 60k€ when you actually wanted to sell.
Or imagine if you are Elon Musk and have to pay taxes on some hundred billion dollars that his tesla shares are worth on paper.
Or imagine if you own a house in London, and the tax office wants you to pay taxes just because you could sell it now for a large profit. Do you sell to pay those taxes (with the added benefit that the tax office can also collect stamp duty from the buyer)? Do you get a mortage, maybe in addition to your current one that you can barely afford?
Every tax office would love to tax you or Mr. Musk on those gains, since they would get the money earlier. And if you start with a loss, you usually don't get money back, so the tax office would always be ahead. (Although I am not sure if UK allows you to substract your bitcoin losses from your other income, which could be considered a money back for many employees.)
So yes, it is usually to your benefit that they don't do that.
Vice versa, if you have a loss, it would be to your benefit if you could get the money earlier. In fact, you could come up with a neat trick and sell your bitcoins that you bought at 60k€ for 20k€, to compensate some other gains and pay less taxes. And to not miss out on potential gains, you just buy the bitcoins back at once. Most tax offices don't like that trick, and UK also seems to have a 30 day wash sale rule for that reason.
As I said, I am not familiar with UK rules, but I doubt they do it differently than the rest of the world. And if you would be allowed to choose, you would always pick what's best for you (e.g. you would not pay taxes on hypothetical gains, but declare losses on hypthetical losses), so it is unlikely that such an option exists.
Nevertheless, there might be UK specific exemption limits where, if you are below those, don't have to pay taxes and maybe also don't have keep track of your gains at all (until you are above and have to backtrack everything).