For anyone stranded, read up on UK261/EU261 and your airlines application process to be reimbursed. As far as I’m aware it’s not likely to happen automatically, you need to initiate the process (and record audio/video just in case proof is needed)
Compensation yeah, only applies if the reason for the delay/cancellation was under the airline's control. Nats down is pretty clearly a reason to reject compensation claims.
That gets them off the hook for even second order effects. No harm throwing in a claim though, there's no penalty for having a claim turned down.
The legislation is on the link below, it might be worth a once-over with your chat bot of choice to understand out if your circumstances are covered: