Advertising on money

Coins can circulate for up to 50 years, usually one side is reserved for a particular image, in the UK this is currently the queens head. The other side is usually the same however often special coins are released to commemorate specific events.
When there are no events to comemorate, why not sell advertising space on that side of the coin?, or maybe design notes to have a small advertising square built in to the design that companys can pay to advertise on?.
I know most people don't really like advertising but this type isn't as "in your face" as most other methods like tv, radio, billboards etc. If the money wass all given to a particular cause - perhaps in the UK it could be the NHS - then I think people wouldn't mind.
It would also give coin collectors something else to collect.