Q:

What is the face value of sovereign and Britannia gold coins?

I have seen claims on the internet saying sovereigns have face value of £1 and £100. These coins therefore have an embedded put option at the strike being the face value. 

A:

Here's the explanation from WikiPedia:

Britannia gold coins contain one troy ounce of gold and have a face value of £100. Gold Britannias also are issued in fractional sizes of one-half, one-quarter, and one-tenth of a troy ounce and with face values of £50, £25, and £10 respectively.

The sovereign is a gold coin of the United Kingdom, with a nominal value of one pound sterling. Prior to 1932 it was a fully circulating coin within Britain's then Gold Standard currency. Today it is used as a bullion coin and is sometimes mounted in jewelry.
I don't know how you'd have a put on something with higher actual value than face, because otherwise the same would be true with the American Eagle $20 and $50 coins.

