HM
You've not mentioned what qualifications/experience you have.
Assuming you are not (yet) qualified, to work in HF in NATS (or most other companies) you usually need two degrees, a first degree (usually psychology, engineering or design for this kind of HF, i.e. 'cognitive ergonomics') and a Masters degree in ergonomics (see
The Ergonomics Society • Careers & Training). There is now distance learning Masters offered by Derby Uni, which makes things a bit easier as this can be combined with an OU BSc/BEng degree. Alternatively you can do a BSc in Ergonomics at Loughborough and not require a Masters. In rarer cases you can get away with another BSc degree alone (e.g. Bournemouth Psychology with Computing; Design) but would then generally still need some experience. Without a degree though, it is impossible.
Unfortunately, at the moment there appear to be no positions available in NATS and in fact some contracting positions have not been renewed. This trend is likely to extend to aviation generally, and consulting. Rail and nuclear should still be OK. In three years or so things should have picked up - assuming you have not got a degree yet? If you have and are looking for a job - rail would be a better bet, but The Ergonomist newsletter is the place for job adverts.
There are other options though in various industries, particularly nuclear, rail, military and consulting (all industries - those above plus chemical, oil and gas, medical, etc).
Let me know if you need to know anything else - I've worked in most of the above to a greater or lesser extent.
HS