ETASSUM48190 | Company Share Option Plan (CSOP): Taxation: Disposal of shares - general
From HM Revenue & Customs · Employee Tax Advantaged Share Scheme User Manual
A disposal of shares is a chargeable occasion for capital gains tax purposes. In computing the capital gain or loss on disposal of shares which were originally acquired by directors or employees exercising Schedule 4 CSOP options, the base cost to be deducted from the disposal proceeds is normally, for options acquired on or after 10 April 2003:
the amount (if any) paid for the option, plus
the option exercise price paid for the shares themselves, plus
any amount that counted as employment income of the employee under section 476 when the shares were acquired by exercising an option.