At an A-CDM airport (such as LHR) the taxi time will not be a blanket value, but will depend upon your stand/parking location, aircraft type, runway in use, and time of year, perhaps also modified by your particular airline. This is the VTT (Variable Taxi Time)
At such an airport the CTOT will be based on your TOBT+VTT+runway holding point buffer (if any). As long as you've called ready within the TOBT tolerance, that's your job done.
Call ready as soon as you are ready, that's all you can do.