GOV.UK
Shares and securities: share identification rules: share pooling
The share identification rules have been subject to significant changes over the years but the usual - and current - approach is been to pool all acquisitions of shares of the same class in the same company. Also, the rules for capital gains tax and corporation tax have differed. The way the rules have changed over the years now has little relevance for capital gains tax but for companies there is still a need to identify shares held at 31 March 1982 or 6 April 1965 separately.
The share identification guidance was revised in 2014 and now contains separate parts describing the rules for capital gains tax disposals from 6 April 2008 and for corporation tax disposals from 1 April 1985. See CG51550+ and CG51600+.