"That said, when I look at the options for the indirects, there is not often a big saving. Perhaps in Y but not in PE or C for obvious reasons."
My experience is the exact opposite - relatively small savings in slave but very large savings if you turn left.
One issue is that many carriers charge almost the same for any class of travel whichever UK airport you fly from (even tho the charges are very different) - you have to go via somewhere else to get real savings