George II reigned from 1727 to 1760. In that long period, proofs were issued in only one year, 1730. Here is a gem example.

During George III's long reign from 1760-1820, many patterns and proofs were produced, mostly from the revolutionary Soho Mint, which utilized the first steam powered coin making machinery in the world. But first, they had to overcome the resistance of the Royal Mint to the new technology before they got a license to produce coins for Britain. As it was a private concern, many patterns and trial pieces were produced during the year shown on the coin (early Soho), later than the date shown (late Soho) , and by WJ Taylor later in the nineteenth century after obtaining the original Soho dies (restrikes). It is very difficult to distinguish the stage at which a piece was produced and usually you go by the state of the dies used in the strike.

As a private concern, the Soho Mint was free to make pieces in different metals or gilt pieces as well. 

Here are some examples, mostly in PR 65, some of which came from the family holdings of the descendants of the original proprietors of the Soho Mint (the Boulton Family).