While it would be great to see TUI offering more destinations from BFS, the fact that BFS is TUI's sole seasonal base, and is operated by a Sunwing aircraft, makes it clear that TUI are not interested in the NI market. A permanent base, or at least a second aircraft, would make destinations such as Cape Verde a more feasible option, but by the looks of things, what BFS has currently got is as good as it gets from TUI.
Aren't Norwich and Leeds seasonal bases?
If TUI weren't interested in NI, they wouldn't be there - especially not with a leased in a/c!
How would a permanent base make it more feasible - as you've just pointed out, it's to do with a/c availability.