NATS only got the contract back 11 months ago, the rot set in under ANS.
Neo380 or MoL pointing the finger at NATS are way off the mark.
https://travelradar.aero/nats-gatwick-airport/
The underlying problem generally with Air Traffic staffing is you have a small number of qualified people doing the job.
Say you need 4 or 5 ATCOs to run a radar bank or tower, to add another for contingency adds 20/25% to cost base. The sums are very different for an airline with 100s of pilots qualified on the same equipment.