Income tax + NI at vest
When RSUs vest, their market value on that day is treated as part of your salary for that period. You pay income tax (at your marginal rate) and employee National Insurance on it, usually collected through PAYE. A big vest can push you into a higher band — or over £100,000 into the 60% trap.
Sell-to-cover and why you're not taxed twice
To pay the tax due at vest, brokers typically 'sell to cover' — selling enough of the vested shares to cover the income tax and NI, leaving you the rest. Your cost basis for the shares you keep is the vest-day price, so you've already been taxed on that value as income.
Capital Gains Tax when you sell
When you later sell the retained shares, only the gain ABOVE the vest-day price is a capital gain. After the annual CGT exempt amount (£3,000 in 2025/26), that gain is taxed at the CGT rates for shares. If you sell immediately at vest there's usually little or no gain, so no CGT.