The British two pence coins issued from 1971 through 1992 are bronze, 25.91mm in diameter, and weigh 7.12 grams. Composition was changed in mid 1992 to copper-plated steel (although there do still seem to have been some bronze coins issued through 2000), which are 25.86mm in diameter and weigh 7.14 grams.
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According to the Royal Mint, an unworn 20p coin weighs 5.00 gm.
Interestingly, its diameter and weight are within a couple of percent of a U.S. 5¢ coin, which is worth about 1/7 as much at current exchange rates.
The Twenty Pence coin originates in the UK so, as the name suggests, it is worth Twenty Pence in the UK.