Without knowing the airline and other details there can be no definite answer, but it may be that their performance manuals don't have N2W figures but only "full length" "Block 85"( or whatever that is called now) etc. etc.
It wouldn't be the first time that this has happened, especially if it is not their home base. Whilst some crews may use their own discretion others are more tightly restricted by their SOP's
If it is the same airline on each occasion then it might be worth contacting their LHR office and enquiring if they have the figures.
One other thought is that sometimes an aircraft will have some piece of equipment not working and the MEL/ADD/DDM book will tell the crew what restrictions are associated with it. These are generally not runway specific, and it may say, for example, that if the Autobrake on reject take off is not working then use the full length, even though in this case it is patently over the top.