BAladdy - Telaviv and Gambia are within reach of 737 - 800 from the UK, and certainly within reach of the max, the issue becomes one of flight time limitations and cost of night stopping crews, if you only do one flight a week you end up spending a lot of money on crew hotels.
As for crew training, whilst the 757 crews would have to retrain to fly an Airbus or a Max, once they’d retrained for a Max then there would be far more flexibility for crewing or rostering as any of the company’s pilots could fly any of its aircraft.