CG41572 | Open-ended investment companies (OEICs): tax regime
From HM Revenue & Customs · Capital Gains Manual
OEICs are treated in practice as though they were authorised unit trusts, (this includes TCGA92/S100 (1) which means that any gains arising to an OEIC are not chargeable gains)
shares in OEICs are dealt with in the same way as units in an authorised unit trust, (this is basically the same as the treatment for other shares, with a special definition of market value, see CG57680+).