Sorry I can't help out much with a UK source, but any full-line house should offer a panel milliammeter in the 20mA full-scale deflection category or thereabouts.
http://www1.rswww.com/ has some if you search around for panle meters. Sorry but their URL scheme makes it impossible to give you a direct link. A bit pricey,but pretty attractive. Also, eBay can be a good source for panel meters.
Remember too, all moving-coil meters are fundamentally ammeters, so you just would change the series resistor on a voltmeter to a shunt resistor in the appropriate size for your needs (and replace the scale).