2018 Bronze 20-P Question

It would be very helpful if not essential to include pictures as well as the weight and size.

I'm not entirely sure how a genuine mint error "bronze 20 pence" would come about. Bronze isn't used for coinage anymore, anywhere, except for Japan and Japan doesn't use The Royal Mint to make coins for it. It might be copper-plated steel, in which case your coin should be magnetic; in either event, the 20 pence coin is made on specially-shaped heptagonal blanks and as far as I am aware, nowhere in the world uses a copper-plated, heptagonal-shaped coin.

There are, however, two ways in which such an object might be made outside of the mint; if these have happened to it, what you have is a "damaged coin" rather than a "valuable mint error".

The first is obvious: someone electroplated it. I'm not quite sure why someone would electroplate a cupronickel coin with copper, unless they were using it in an electrochemistry demonstration.

Second, normal 20 pence coins are made of cupronickel - an alloy of copper and nickel. If this alloy is placed in certain environmentl conditions (chlorinated fountain or pool water, for example) and left there for several weeks, it can take on a coppery appearance. This would qualify as "environmental damage".