Some of them went for a walk along the full 3+ miles of the main runway yesterday just for the fun / novelty of it!
Herein lies a consideration: when the time comes again to fly, it's going to take an age to unseal, re-boot and uncover the engines and get everything back up to speed, so at some point everyone will be waiting on the engineering at all the airports, and don't forget they only do a 2/3 (of 24hrs – there's no grave yard shift) shift and there aren't that many to go 'round, and not to all the smaller airports where their planes are parked up such as Prestwick which has no permanent BA maintenance dept.
Also, many airlines only lease their planes, even BA don't own their own engines, they're leased from RollsRoyce, so the daily bills still need paying!