From UKCAA-AIC 151/1993 :-
V1 must be respected as the latest speed by which a decision to stop must have been taken to assure a stopping capability within the distance available on a limiting runway.
It is not the speed called at which the PF decides to reject the take-off and use the appropriate procedure. In other words when V1 is called (human or automatic voice) the decision must be to GO.
The JAR accelerate-stop definition differs from the FAR definition in that the JAR includes the stopway in the calculation, whereas the FAR determines that the aircraft will stop on the runway itself.
In JAR's a 1.5 to 2 second "thinking time" is included in the V1 calculation.
If V1 is called early (beware the lawyers if is is found to be a factor in an accident!) then it is not guaranteed that the a/c will achieve lift-off before the end of the runway, following a power-plant failure nor achieve the 35' screen height (15' if wet or contminated conditions).
V1 and its' definition has been argued since long ago and will be argued for a long time yet. The 'sense' in the GO decision, when above Vmcg but below V1 may be obvious but ill-advised. Stick to SOP's and if you don't, be prepared with a good defence!!