Isn't it a little bit obvious? You can not pay taxes for seeing the price going up while you are holding, it would be irrational.
You are OBLIGATED to pay taxes when you withdraw those bitcoins, if you cash them all by your bank, at the moment that you get called by the bank to justify why and how did you got those funds.
You don't have to pay tax on it if it increases in value and you just hold it in crypto
You have to pay tax on the "profit" made when you sell it back to GBP
Something like that is happening on my country, when you want to cash out, the bank is going to call you to ask you how did you got those bitcoins.