Yes - they are an issue. Currently about 1 in 30 pound coins in circulation are counterfeit - so that's about £45 million in circulation (source http://www.theguardian.com/news/datablog/2014/mar/19/five-th...).

I suspect generating them is less effort that many people think. This guy http://www.dailymail.co.uk/news/article-502292/Counterfeiter... - was convicted of creating 14 million fake £1 coins and the technical side was basically a one man operation.