Never used it but can't they bring double deckers from the top of the hill then via the bypass?
Seems bizarre how the Mill Road P&R is all over the shop when it ran for all those years. Tallies with the experience a London mate of mine had when he used it for the Elton John concert.
Our bus had a spot of bother getting up the hill because of the big backsides of the giant 4x4s. Not sure a double-decker would find it easy. To get up the hill in the first place, they have to go under the bridge. If they avoided the bridge, then they would have to go up and down the hill, and do a turn. Impossible, because of all the parked cars and the lack of space.