I think the agent's correct answer is to reserve some seats on TLV-LHR and then send the booking to the Fares team to calculate the fare difference manually. This should take them a few days but unfortunately you may well need to chase it a few times. You can guestimate the fare if you go though your original ticket's breakdown and compare it with TLV fares but my guess, without actually doing the exercise, that CW from TLV will be fairly pricey, whereas World Traveller from TLV may be a more reasonable fare.