RE: Tax year implication1 Apr 2021 11:01
CGT is only calculated on the realisation of a profit or loss which is crystallised on sale.
If you buy 1000 shares at £1 on 1st April and they are worth £2 on 5th April but you haven't sold - that isn't a taxable gain, similarly, if they were only worth 50p, that isn't a taxable loss - the gain or loss happens when you sell them and the date of sale dictates when you have to declare to HMRC.
The scenario you are talking about suggests that if you buy shares for £100K and sell them for £400K, you don't have a CGT liability if you've then reinvested that £400K in other share purchases which is wrong, you've made £300K and you pay tax on it.