I haven't looked up the stats but BA252 NAS-LHR has a poor anecdotal reputation for delays / cancellations. 36 hours delay for me a few weeks ago, with various pax from different days all grumpy and milling round the airport.

Staff at NAS were rolling eyes and bad-mouthing BA so definitely didn't seem like a one-off.

On your specific question, I don't see why the GCM hop being cancelled necessarily means the NAS-LHR will be.

Good luck!