Here's what the AIP says:
Airport - Collaborative Decision Making (A-CDM)
(1) TOBT/TSAT
+ (aa) Pilots should take note of the TSAT which they receive from their AO/GH or ATC and comply with it;
+ (bb) If TOBT or TSAT can no longer be met, at any time, then TOBT must be updated by AO/GH;
+ (cc) Pilot should ensure that the flight is ready to depart at TOBT (window of -5 to +5 minutes).
(2) Start Request - Heathrow Delivery
+ (aa) Pilot should report ready to Heathrow Delivery at TOBT (window of -5 to +5 minutes);
+ (bb) ATC will then approve start or in the case of a delay will advise the TSAT;
++ (i) Pilots to monitor the frequency from this point, as TSAT can improve up to TOBT;
++ (ii) Start approval will be issued, based on TSAT and the prevailing traffic situation;
++ (iii) Pilots will be informed of an ATC delay to TSAT in excess of 5 minutes.
+ (cc) If at TOBT + 5 minutes ATC have not received a start up request the aircraft may lose its position in the sequence.
++ (i) ATC will advise the pilot that a new TOBT is required;
++ (ii) Once the new TOBT is entered the flight will be re-sequenced according to a new TOBT, with a subsequent delay;
++ (iii) The aircraft will not be allowed to depart until a valid TOBT is entered and revised TSAT given and complied with.
(3) Pushback Request - Heathrow Ground
+ (aa) Pushback/Start clearance must be requested from Ground no later than 5 minutes after being transferred from Delivery;
+ (bb) If unable to meet this constraint, the aircraft will not be allowed to pushback. A valid TOBT must then be issued by the AO/GH and ATC will then issue a revised TSAT.
(4) Remote Holding Request
+ (aa) If an eligible AO is aware of a CTOT and wishes to take the delay on a taxiway rather than on the stand, then they should contact the Tower Supervisor by telephone to arrange it;
+ (bb) In this instance, the TSAT will be adjusted to allow the aircraft to be transferred to GMC earlier for the remote hold.