I support NATS controllers too; it's their HR people who are to blame for the shortage.
When I came up for retirement,(at the NATS 'standard' age of 60) I offered to stay on part time (20 hrs gross per week) as I knew my unit would be 'temporarily ' short of certificated controllers until more could be trained, (they wouldn't post my 'replacement' in until I actually left and it would take at least 6 months to train them up!) but the terms offered to me by HR were unacceptable, plus they told the pension people not to pay my pension as I was 'not retiring', something they had absolutely no right to do. 10 years on now and I understand my old unit is still short of staff.
I know of another retiree (not from my unit) who was treated a similar way and another from my old unit who was made 'an offer he couldn't refuse' to take early retirement at 55,