[rfugger]

According to this BBC Four documentary (found in another comment)

https://www.youtube.com/watch?v=vZ_a21fPkYM

the engineers wanted to apply for a patent once the British government stopped funding their work, but the government classified their work as secret, preventing them from doing so.

EDITED TO ADD: Apparently the original secret classification expired in 1993, at which point they filed a patent which ended up being owned by Rolls Royce. Their current work circumvents that patent.