Just how do you expect FR to make a profit on £20 return flights to some of the most expensive most congested airports in the world with some of the highest tourist taxes in the world? It would be £26 APD before you've even stepped on the plane.
TLV isn't served because there simply isn't sufficient demand to provide a regular service that will deliver the necessary yields. TLV flights are also costly in terms of utilisation as the plane won't have time to fit any other flights in that day and the charges at TLV are fairly high. Better returns could be made on 2/3 rotations to Spain.
Making money is what it's all about. It's not about having exotic destinations or the most diverse route portfolio.
The market decides what will and what won't work. Not the airlines. It's all well and good getting £20 returns but as soon as whatever subsidy that allows such prices comes to an end, so does the route.