Ian Stirling wrote:

However, as this is a UK d-i-y group, I was wondering how it's illegal in
the UK.
Obviously it'd be a crime in the US, and they'd want to prosecute you,
and perhaps even extradite.

Dunno. Forgery is apparently defined in terms of 'making a false
instrument'[1] - so, while there may be more specific statutes about UK
currency, the general case of making fakes of things of monetary value
is covered. However, the wording (from just a quick Google, and no
background knowledge AT ALL), goes on to say 'with the intention that he
or another shall use it to induce somebody to accept it as genuine',
*and* that they stand to gain some advantage (I think). So you *might*
get away with making convincing-looking 'play money' to, say, shower
someone with at a wedding... but before you do, better find out from a
more authoritative source than some unrelated newsgroup ;-)

[1] There seem to be multiple offences, covering various stages of
forgery - 'making a false instrument', 'copying a false instrument',
'using a false instrument', 'using a copy of a false instrument'(!),
'custody or control of false instruments and manufacture, custody, or
control of equipment or materials to make them'. This from the Crown
Prosecution Website at

http://www.cps.gov.uk/legal/section8/chapter_c.html

Stefek, following the old dictum that if it's nicked from one source,
it's plagiarism; but if it's nicked from several, it's Research ;-)