The practice it blocks was selling on 5 April and buying back on 6 April to reset base cost using the annual exemption. Matching now runs same day first, then acquisitions in the following 30 days, then the section 104 pool.
Workarounds that remain include bed and ISA, selling and repurchasing inside a stocks-and-shares-isa, bed and SIPP into a pension, or repurchasing in a spouse's name. Each has its own consequences and limits.
It is the UK cousin of the US wash-sale-rule and the Canadian superficial-loss-rule, but the mechanics differ: the UK version matches forward only, not 30 days before.
General information about the United Kingdom, not tax advice. Rules change and depend on your circumstances; take professional advice.
Related: uk-capital-gains-tax, stocks-and-shares-isa, wash-sale-rule, superficial-loss-rule, hmrc