This is the TAF for Heathrow
TAF: EGLL 141703Z 1418/1524 21012KT 9999 -RA BKN012 TEMPO 1418/1508 6000RA BKN008 PROB30 TEMPO 1418/1419 21015G25KT PROB30 TEMPO 1422/1505 4000 DZ BKN004 TEMPO 1505/1508 20015G25KT BECMG 1508/1510 20019G30KT BECMG 1510/1512 BKN018 TEMPO 1512/1524 8000 RA PROB40 TEMPO 1512/1524 4000 +RA BKN012 BECMG 1514/1517 20028G42KT
The bold bit is likely the reason.
Shorthaul aircraft (320s...) crosswind landing limit is 38kts. That gives a crosswind component of 39kts. Out of limits for them. 747 has a wet crosswind limit of 25kts for landing I believe (?). That’s them out. If the forecast is correct then 100%, the wind will exceed limits at points during that forecast validity.
They wouldn’t cancel flights for the sake of it but to run a full programme with that forecast? Not a chance with how busy Heathrow is.