Entrepreneur Relief allows gains on qualifying assets to be taxed at only 10%. To qualify for this relief, you must hold a minimum of 5% of the company's shares for a continuous three-year period prior to selling.
Regarding the valuation of minority shareholdings, there aren't fixed formulae but guidelines exist. Generally, a discount is applied due to the lack of control and marketability. For shareholdings up to 25%, a discount range of 50% – 70% is suggested if no dividends are paid; for those between 25% to 50%, a 35% – 40% discount might be applied. The exact discount rate would take into account the specifics of the shares and any additional rights they confer.
Given you meet the shareholding criterion, you could qualify for Entrepreneur Relief. For the valuation of your shares, while there's no set formula, generally recognized discounts for minority interests can be referenced for negotiation or independent valuation.