As I've posted a couple of times before, I submitted a compensation claim last year for a cancelled flight. LGW-IOM May last year.
The flight was cancelled as the aircraft was late arriving back into LGW ( around 1945 ish) after delays picked up during the day on European rotations. Probably mostly ATC slot delays but some weather exacerbating these.
Being in the trade, I could prove that the aircraft COULD have flown the LGW-IOM leg, but there may have been difficulty in flying the return due to the IOM Airport latest possible cut-off of 2300.
As they tend to, Easyjet decided not to risk the aircraft and crew getting stuck and cancelled both.
I provided all this information first to EasyJet and was denied. I therefore took the claim to ADR (the independent resulution agency) :
https://www.caa.co.uk/passengers/resolving-travel-problems/how-the-caa-can-help/alternative-dispute-resolution/
They studied my claim and I was still denied. It would seem that if any ATC slot delay is incurred or weather is a factor, even the independent resolution body is likely to support the airline.
That's not to say don't try.... Good luck!