Fixed wing transits very rarely happen, and they never orbit over or near the threshold waiting to cross.
The only fixed wing transits I've seen have been under the control of Gatwick Director and crossed close to the field during periods of light inbound traffic. Radar basically engineers the spacing so that the crosser passes to the east of 26 or west of 08 thresholds, so that we in the tower have a clean climb out and continue to launch departures unrestricted.
One other possibility is that you may have seen fixed wing inbounds to Valence field, basically on the other side of boundary fence, north of 08R threshold. They must approach from the north, be familiar with the site, have written permission, maintain visual contact with Gatwick’s traffic, and not balls it up! It doesn't happen that often, and for us in the tower its no real problem.
As for 'lost' Redhill traffic; twice I've been warned by Radar that there's one tracking a bit too far south and had to stop departures, and once, I've given avoiding action to a B744 because a micro light decided that the quickest way from Brighton to Redhill was up the M23!
....and yes, 2.5A is 2500ft QNH.
I'll leave the bit about IFR transits to Warped!
