It goes without saying that counterfeiting coins is illegal. Sir Isaac Newton when he was Master of the Mint, had people hanged or tortured for illegally producing coins. So, don't do it, ok!
That said, coins are very useful as calibrators for 3D printers. They come in various shapes, sizes, and thickness. More importantly, it's really easy to compare a 3D printed coin with one in your pocket. Or, even, just by looking at them.
The Royal Mint maintains openly available specifications for all UK coins. Predictably, they don't have the coins in a readily available 3D format 🙂
The faces and designs of the coins are also not available. Using a high resolution photo of the designs could be used to generate a height map suitable for printing - but I'll leave that for version 2!
One interesting thing to note, the 20p and 50p are not heptagons and pentagons as many people think. They are Reuleaux Polygons. The mathematics behind them is a little to taxing for a hungover Sunday morning, so I used Dan Newman's freely available OpenSCAD template from Thingiverse.