I presume that BA are moving 767-300 onto the Tel Aviv route as passengers, in particular business passengers, are dropping due to the risk of terrorism and the financial burdens that have been placed on companies in the past months. TLV was previously served with 767-300 in a two class config until BA decided to use the 777 from LHR, offering a three-class layout. However, I would guess they just cannot fill the aircraft.
Alternatively, it could be due to the fact that BA are retiring their 747-236Bs and switching 777 onto the routes previously served by these aircraft. Also, they are not deploying 763 on European (Düsseldorf, FRA, Stockholm, Nice, Munich, Milan, Rome, Zürich) routes as much as they used to, so they are maybe using the 763 to fill the routes that they have taken the 777 away from. It would certainly make more sense, in the long run, to fly a fully-loaded 763 rather than a 777 that is only 2/3 full.
Anyway, they are retiring a number of 763 also which is a sign that they are really having problems filling their larger aircraft.