security experts and mathematicians in the various intelligence services and academia, many of them amongst the best in the world
That's very unlikely.
I just went to the GCHQ careers site. THe first position I clicked on was for an Intelligence Analyst - quite an important role[1]. they're offering a bit ove £25K.
Applied Research offers less than £28K.
At these rates, they might get a few gung-ho patriots, but the real talent will be earning ten times as much. It is likely that the "best in the world" is nowhere near Cheltenham,
Vic.
[1] The job description states "Analysts are at the heart of GCHQ’s mission - turning data into the critical intelligence that helps to protect the UK. Our analysts work with complex data, understanding and interpreting it to find the crucial intelligence within it."