Those were the days when cargo operators using Britannia's seemed to be two a penny but I think the reality was that it was spread over a few years. Was the Invicta one cargo or passenger?
22 RAF Britannias were sold to civil operators in 1976 and a majority ended up at Luton at some time or other.
Invicta ceased passenger operations when it got rid of the Vanguards. The Britannia was a 312F.