https://www.theguardian.com/money/2014/mar/19/cost-new-pound...
Seems like there are £45m worth of counterfeit coins in circulation, but the switch may cost over £100 million.
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EDIT: The percent of fake coins in circulation appears to be in flux, which makes me wonder what methods they use to estimate the fakes in circulation, if they use controls like injecting known fakes into the count to see if they are spotted, if the sample is truly random, the maths is correct, etc.:
1% 2004 [1]
3.03% May 2014 [2]
2.55% May 2015 [2]
Of note is that the Royal Mint states in the 2016 annual report that the last survey for fakes was in May 2015, but the report was issued in July 2016, which means at least as of that date they failed to do the annual survey; or at least publish the results.[3]
[1] http://www.telegraph.co.uk/finance/personalfinance/10707540/...
[2] http://www.royalmint.com/discover/uk-coins/counterfeit-one-p...
[3] http://www.royalmint.com/~/media/Files/AnnualReports/ar_2015...