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)
Eh, I tried in 2017 with BA when their system went down and received nothing. There was magically a Black Lives Matter protest at Gatwick (or maybe LCY, can’t remember) on the same day, which isn’t covered apparently.
You won't get compensation for ATC issues, but you will be reimbursed.