I accept that I am wading out well beyond my depth here; but I would have through that any existing VS slot pairs would be optimised (as closely as can be done at LHR) for turning around a widebody, and wouldn't be overly suitable for maximising the usage of an A320/A321 on a short haul operation. Unless of course, VS are jiggling slots to create new pairs that are better suited as sub lets.
JAS (now paddling back to the shallow end)