9-car Pendolinos which have not yet been converted have a First Class Coach G (which is not declassified).
9-car Pendolinos which have been converted (and there aren't many yet) don't have a First Class Coach G. It has been converted (i.e. the seats ripped out and replaced) into Standard Class accommodation.
11-car Pendolinos all have a First Class Coach G (which is not declassified).
So no matter what the rolling stock (with the exception of the few Standard Class only Virgin Trains services, such as the 18:57 Euston to Manchester), you will not be able to occupy a First Class seat with a Standard Class ticket unless the train happens to be exceptionally busy. (Of course, this being a Sunday, you could always purchase the weekend upgrade.)