Don't recall Malta being used for Viscount training in the 60s although they operated the London-Rome-Luqa as well as Luqa-Tripoli-Benghazi and Luqa- Catania for Malta airlines. My V800 conversion was done at a deserted Stanstead
However, from 1964 Malta certainly was used for the massive Vanguard training programme as the Hamble graduates came on stream. This was partly because of the excellent winter weather statisitcs and partly due to a very attractive deal on landing fees. As before mentioned the training aircraft could be seamlessly integrated into the passsenger schedule.
As increasing traffic and increasing landing fees began to rule out old favourites like Stanstead and Prestwick, then continental options such as Beauvais were used for 757s.
When Trident automatic go-arounds from 14 feet were being practised at Prestwick, ATC sometimes could not tell whether the wheels actually touched (with consequent higher charge!)