Ofcom has recently started issuing G5 + 3 letter callsigns to radio amateurs
In the last century, from 1966 until 1981, G5 + 3 letter calls were issued to foreign citizens as Reciprocal licences:
G5AAA-G5DZZ were used for Class A's and permitted operation on HF bands
G5NAA-G5ZZZ were used for Class B's and permitted operation on 144 MHz and above
From 1981 reciprocal callsigns no longer had a separate Prefix, instead they were issued from the standard Class A (G4) and Class B (G6) call blocks.
In the 21st century Ofcom has, when requested, reissued a few of the old G5+3 letter callsigns to their previous holders, but recently Ofcom have started issuing new calls from the G5 block.
One of the beneficiaries has been the G-QRP Club, which is devoted to Low Power operation. That club now have the callsign G5LOW - http://www.gqrp.com/callsign.htm