Plenty of people have left Ryanair without serving their notice period. I flew with a captain who received a job offer on a turnaround down route, handed his ID and iPad in on return to base and was never seen again. It’s not the worst advice in the world in the current climate, and airlines have bigger fish to fry right now than chasing an FO for not serving their notice.
The best advice though is don’t go to BA. Their true colours have been shown in their treatment of pilots and cabin crew with signed contracts to join. From what I gather, the goodwill doesn’t improve after joining either.