We know 1 in 30 £1 coins are counterfeit.
Take the amount of £1 coins in circulation. Sum the value. Divide by 30. Equals the market in counterfeit coins.
From http://www.royalmint.com/discover/uk-coins/circulation-coin-... £1 are 1,671.328 £m / 30 = 55.71 £m
The market in counterfeit coins is therefore over 55 million pounds. This is, however the cumulative, total value of the market and not per year.