The Defiant was designed to fly alongside German bombers and hit them from their defensive blindspots. This wasn't a totally stupid idea when the UK was out of range of German fighters, the rapid fall of France wasn't planned for of course. Having said that the Defiant was never going to be as effective an interceptor as a Hurri or Spit, so it was a waste of effort.
During the BoB the RAF could have kept the Defiants covering the north of England releasing Spits and Hurris south, rather than sending them into the 109 infested English Channel to get slaughtered.