I know the railway station is close, but if public access were to be from the Gannet side as opposed to the terminal side, it'd be a bit of a hike for some... 2+ miles.
At the last Prestwick show I attended in the late 70s they ran shuttle buses from the station round to the east side of the field.
My other recollection was that during the show there were some normal ops that briefly interrupted the show from time to time. (More to see
)