(3) For the purposes of this section,  “short fire arm " means a firearm either with a barrel not longer than 30 centimetres or whose overall length (excluding the length of any detachable component) does not exceed 60 centimetres. 

That's 12 ins in old money and 23.5 ins over all length, with a wire coat hanger or "arm brace"

The UK legislation is the 12/24 rule

UK’s 12/24” (Large Firearm) law with a 12” barrel and fixed-grip extension (brace) to make the minimum 24” required length! 

Ya know looking at this... The UK long-barrelled pistols are licensable here as they fit the same legislation. God awful contraptions,but if it got more people back into CF shooting?