Sounds like your feeling unwell started before the hot weather started, but most people are finding themselves feeling tired due to heat. I can still remember the 3 summers I spent working as a nurse in London, lomng before I had diabetes. These days, I wouldn't want to be in London more than essential - although coped fine with Games Maker shifts during Olympics & Paralympics. With diabetes, keeping blood glucose levels as near normal as possible, by a combination of exercise, sensible eating and medication is vital - if you can achieve that, then there's no reason why you should feel unwell, nor be any more prone to getting infections or suffering worse than anyone else if you do catch something.
While flopping in front of TV is appealing, if you can make use of the cooler air of evening (or getting up early to use morning coolness) for a walk / bike ride etc, then overall you will feel better.