This is the obvious problem that you'll run into where there are different levels of service at one airport ... and, I guess, it's why the CAA discourage it.
How to fix it ...? Use the correct callsign suffix at all times (both TOWER and RADIO) and if a pilot appears unsure, call the pilot and ask for confirmation the he or she is aware that they are getting an ATC or AG service.
Drawback is that many pilots won't know the difference anyway!