If you know the 'wrong' people you can buy £100 of pound coins for £40 (if they don't work in vending machines) or ~£80 (if they do work in vending machines).

This is a thing in black market channels, so, for the junkie with nothing in life except where the next high is coming from, you can imagine that they can spend all day passing off these coins to get legitimate cash back, for that next high and for that next batch of counterfeit coins to pass off.