UK tax rules advice

I’ll preface just by acknowledging I’m aware that every transaction is a taxable/ tax deductible event

I started buying/ selling/ swapping different tokens in 2020. Ultimately consolidating all my crypto assets into BTC. If I had invested say £50,000 and in 10 years time it turns out that I sell and the final value is also £50,000 is that too long a time frame to evidence that there was zero pound sterling profit. Or would I be exposed to having to pay tax on the £37,700 considering the UK allows £12,300 p/a tax free capital gains?

Further more, if the assets dropped to £20k for instance after 10 years would I have to pay capital gains tax on £7.7k as so much time has passed? Or it gained value to £100k, is the £78.7k taxable if with the initial £50,000 invested. I remember vaguely reading a while back about a 3-4 year time frame of activity you can declare your original investments and after that timeframe it is disregarded?