UK registrations were very much issued "next one alphabetically in sequence", from G-AAAA in the 1920s, they only moved on from G-AZZZ to G-BAAA (one of the Court Line Tristars) by 1973, so there was loads of capacity. Nowadays only a minority still take the next one, you can have what you want - and pay for it. If you made a major purchase of new aircraft, normally just BOAC/BEA, you could get a block of 26 ending A-Z, but again it would be the very next such block in sequence. The Vanguards were thus all G-APEA and upwards.
Just a couple were skipped. In best civil service tradition, the one which alphabetically followed G-ARSD is blandly described as "Registration Not Allocated".