ITA is not giving you accurate information, unfortunately. The present availability on those flights as seen in London is—
Flight Stops Depart Arrive Aircraft Reliability Available Classes
LY 318 0 LHR TLV 772 Su,Th,Sa F6 AL PL C9 I5 D2 ZL JL RL XL Y9 S9 M9 Q9 B9 K9 V9 L9 H9 NL GC OC UC WC EL
24/12/15 22:30 25/12/15 5:20 75% / 15m
LY 315 0 TLV LHR 772 Su,M,T,W,Th,F F6 AL PL C9 I9 D8 ZL JL RL XL Y9 S9 M9 Q9 B9 KL V1 LL HL NL GC OC UC WC EL
03/01/16 9:05 03/01/16 12:35 52% / 28m
In both cases, the W class is closed to reservations (WC); therefore the fare ITA selected (LY WUKRT5) is no longer valid over the sectors.
If you wish you may report this as a bug to ITA Support. I have found the support team quite helpful in respect of other problems I have had with ITA.