I think Britannia started off in a hangar built by Luton Corporation who built it to attract an airline to the airport. Around 1970 they moved into their newly built hangar 89 next to the terminal. Court Line then moved into their old hangar to maintain the Tristar's.
At some point while keeping hangar 89, which was too small for the 757 and 767, they moved back into hangar 65 which was rebuilt.
Years later Easyjet moved into hangar 89 leaving leaving Britannia with just hangar 65.
Britannia built H89 around 1969 when the 737 entered the fleet. 757's did fit easily inside (at an angle) and it was common to see two undergoing maintenance at any one time. Cut-outs in a section of doors were made to accommodate the 767 with the tail outside but they rarely were seen in that hangar. The exiting of H89 had nothing to do with the withdrawal of the 737's in 1993. It was part of a cost-cutting exercise around 10 years later (although the lease was almost-up and the airport were keen to get rid of the EZY tent on the East apron!)
The original single bay H61 was built around 1983 prior to the 767 entering service in 1984. It was extended to the current two bay hangar in the early 1990's by demolishing the old Court Line hangar.