The only way to fly for a Summer month which embraces the whole of 5 week-ends is to buy a transponder. The cost & not even installed is a whopping £1590 for the minimal version by TRG.
Man, this is the 21st century!
You should consider yourself lucky you've been able to fly so close to Gatwick without even a transponder.
Over here we have a Mode C requirement at 30nm around the major airports as standard.