I'm sure that will be the case during the peak. During peak time most trains towards Euston run late from Harrow or they did last time I worked around there.The same way they cope now - they use SDO (front 7). I can't see why UDS would be a problem, it isn't a problem anywhere else.
UDS is unit deselect, the simple form of selective door opening LM use where you can open only the doors in the units forward of the cab you are in.
I expect the reason peak services do not call is simply because they are already overcrowded without adding Wembley passengers to the mix. There might also not be room in the peak timetable for it, as more trains run than off-peak.
Neil
Of course numbers of trains running won't be an issue on Sundays.