Hey Jasper...
a couple things going on here.
The horseman slaying the dragon is St. George, the patron saint of Britain. This design has been used on various British and Commonwealth (Australia, Canada, etc...) coins since the early 1800's. In modern times, it was used exclusively on the gold sovereign:
On yours, the date 1951 was used possbily to avoid counterfieting charges, as the sovereign was not minted in that year. For that matter, no coins were minted in 1951 that looked similar to yours.
On the other side of your piece, the bust is of Queen Elizabeth II and the same thing going on with the date. Her image did not appear on coins until 1953. At the time she took the throne, she was 27 years old and the bust used on coinage from 1953-1964 looked like this:
The image of the queen on your piece, is a later bust used from 1965-1989. For example here is a Canadian dime that is similar to the one on your piece.
For interest sake, the D.G. Regina inscription, stands for the latin, "Dei Gratia Regina" which means, "By the Grace of God, Queen"
Hope this helps.