The Sovereign
A gold Sovereign weighs 7.98g and is 22 carat, so it holds 7.32g of fine gold. It measures 22.05mm across and is UK legal tender with a face value of £1. The modern Sovereign has been struck since 1817.
Half Sovereigns and Quarter Sovereigns hold a half and a quarter of that gold.
The Britannia
A one-ounce gold Britannia holds 31.10g of fine gold. It measures 32.69mm across and is UK legal tender with a face value of £100. Since 2013 it has been struck in 999.9 fine gold; Britannias struck from 1987 to 2012 are 22 carat and hold the same weight of fine gold.
Britannias also come in half, quarter and tenth ounces, holding 15.55g, 7.78g and 3.11g of fine gold.
How they compare
A one-ounce Britannia holds a little over four times the gold of a Sovereign: 31.10g against 7.32g. A Sovereign and a quarter-ounce Britannia are almost exactly the same size; the Britannia holds a little more gold.
A holding in Sovereigns divides into smaller steps, so part of it can be sold, or passed on, a coin at a time. A holding in Britannias puts more gold into each coin. A holding can combine the two.
The tax is the same
Both are UK legal tender, so a gain when you sell is generally outside Capital Gains Tax, and investment gold coins carry no VAT. Tax rules can change and how they apply depends on your personal circumstances.
Choosing between them
That depends on how much you are putting in and how you might want to sell or pass on the coins later. A written recommendation names specific coins, with the price of each; the decision is yours.
Capital at risk. The price of gold can fall as well as rise, and gold pays no interest or dividend.