Why postwar Britain might retain flying boats

What circumstances might have kept flying boats in Royal Air Force service after the war?

A need for dispersed maritime aviation that could survive attacks on fixed bases, followed by a workable British or joint replacement design.

The operational requirement comes first. Britain would need to decide that water-based aircraft still provided useful dispersal and survivability if fixed airfields were threatened by nuclear war. Retaining the capability would then require rejecting an unsuitable Saunders-Roe path or developing a more practical domestic aircraft. A joint project with Japan is conceivable, but simply buying a Japanese design is less likely than preserving a British industrial role.