It might be much more effective to ask them to only take part in or start defense-related activities at NSA or GCHQ.
Even if they only work at protecting the US and UK from foreign agencies, that will still result in mostly securing the whole world since hardware and software is used worldwide.
It's also more realistic since it doesn't involve destruction of those intelligence agencies but merely retasking over time.
The best defense is a good offense. Why secure the whole world when you could just secure yourself and your allies, and keep everyone else full of holes you already know about?
Even if they only work at protecting the US and UK from foreign agencies, that will still result in mostly securing the whole world since hardware and software is used worldwide.
It's also more realistic since it doesn't involve destruction of those intelligence agencies but merely retasking over time.