All the patches I've seen in photos have more or less been in line with the "Approximately 18 inches square" directive, and applied and positioned in the appropriate place for the aircraft type - that is, starboard wing on fighters, rear fuselage deck for trainers, and rear fuselage for 'communications' types or multi-engine/multi crew position other types.
The pic of Mamedoff's 609 Sqn Spit, which I have in a number of books including Chris Goss's coverage of the Squadron in the late BoB, appears to show a BDR patch, and it is much smaller than the 'regulation' size, as well as appearing darker than the expected tonal reproduction, even allowing for variations in printing processes.
Just going off photo evidence alone, I've seen more BoB period aircraft without the patch than with, but, of course, this may be due to relatively high turn over of aircraft, damage, wear or loss of the patch (especially if not a painted variant), and lack of time, with more pressing requirements during the period, to maintain or replace them.
Generally, I would expect a fighter in the front line up to around early July 1940 to be more likely to have the patch, but after that, as the Battle progressed less likely.
If I could just find the booklet, and another reference source, I'm fairly certain there is brief mention of the patch, and it's lapse of use and eventual abandonment (along with other 'binned' regulations) and the reasons. It's this info, based on wartime accounts not necessarily related to this specific subject, on which I'm basing my replies on, along with a small dose of logic!