Gold coins that are UK legal tender are exempt from capital gains tax for UK residents. That means the Britannia, the Sovereign (full, half, double and quintuple), and the Royal Mint’s other legal tender bullion series such as the Queen’s Beasts and Tudor Beasts. It does not matter how much you make on them.
What is not exempt
Gold bars, Krugerrands, Maple Leafs, Eagles and every other coin that is not UK legal tender are chargeable assets. Gains on them count towards your annual CGT allowance in the year you sell.
Why it matters
A Britannia costs slightly more than a bar of the same weight because of minting. For a small holding that premium is the only difference. For a holding you expect to grow, the exemption can be worth far more than the premium.
This is general information, not tax advice. Check your position with an accountant.