The UK's 6 x Gnat F1 fighters were used by the Ministry of Supply as development aircraft, but as far as I'm aware, were never used by the RAF.
The Midge was even smaller and lighter than the Gnat fighter, with an MTOW of only 4500lb and was powered by a Viper engine. The Gnat F1, though with a wingspan only 1ft greater than the Midge, had an MTOW of twice that and was powered by an Orpheus with about 3 times the thrust of the Viper.
I remember Douggie Mee talking about the Mona incident when I went back to do a Hawk 'refresher' course at Valley in 1980. If I recall correctly, the engine stagnated during a roller and the brakes weren't up to stopping the little monster - so it went for a stroll across country off the end of the RW.