From my man in the know:
"For some time we have felt that the facilities in T3 are somewhat inadequate for an Intercontinental wide-body flight arrival/departure. For instance the refreshment facilities available are not sufficient to cope well with even 'light refreshments' for almost 300 passengers. On any major delay there is no facility to provide a main meal at all. We have to go through the lengthy procedure of bringing passengers back through Security and taking them to T1 to feed them there. There is also only one stand (55) capable of taking aircraft with the wingspan of the A330/B777 (44 on the corner is also possible but not ideal).
All that, coupled with protracted negotiations for the renewal of our handling contract with BA, led us to the decision that now would be a good time to make the move. So from 14th May, it's T1 and Servisair.
The BA codeshare on EK017/018 is not affected and, as with the EI/BA codeshare on MAN/DUB, BA ticketed passengers will checkin in T1."