Capital Gains when selling foreign shares

Over my my working life I was periodically given company shares (UK resident working for an American Company). They are all now in a lump sum controlled by Interactive Investor. If I were to sell some, above £3000 (equivalent) how would the CGT be calculated?

Hi, 
This forum is for general queries only and is intended to help you self-serve. We are unable to provide specific advice tailored to individual circumstances. Please see the links below. 
Capital Gains Tax: what you pay it on, rates and allowances: Work out if you need to pay
Tax on foreign income: Reporting your foreign income
Thank you

OK, who/where can help determine gains (as per the link), when shares bought at different times are now combined in one lump sum? Thank you.