The rule
Capital Gains Tax does not apply to sterling currency. Britannias, and Sovereigns struck since 1837, are UK legal tender, so a gain when you sell them is generally outside Capital Gains Tax.
The face value on the coin, £1 for a Sovereign and £100 for a one-ounce Britannia, is far below the value of the gold it contains. What matters for the tax is the legal-tender status, not the face value.
Why bars are different
Gold bars are not legal tender. They are a commodity, so a gain on bars can be chargeable to Capital Gains Tax in the usual way. The same goes for coins that are not UK legal tender, and Sovereigns struck before 1837 are treated differently again.
VAT
Investment gold coins are exempt from VAT in the UK, and Sovereigns and Britannias both qualify, so there is no VAT on the price you pay. Investment gold bars can qualify for the same exemption, so VAT is not usually what separates coins from bars; Capital Gains Tax is.
What the exemption does not cover
Inheritance Tax: coins form part of your estate like other assets, and Inheritance Tax applies in the usual way.
Trading: if buying and selling coins amounts to a trade, the profits can be taxed as income instead.
Tax rules can change and how they apply depends on your personal circumstances. We are happy to walk through this with you. Nothing here is personal tax advice; for that, speak to an accountant or a tax adviser.
Capital at risk. The price of gold can fall as well as rise, and what you receive when you sell depends on the price on that day.