During periods of extreme heat and drought it is important to limit the amount of time you spend outdoors, but the garden will need a bit of attention!
The best thing you can do is to collect waste water from indoor usage (eg, from washing up or rinsing dishes) and to get out early in the morning for watering the plants that need it most. Collecting waste water is a good thing to do anyway, but it's especially important during hot dry weather when national supplies may be stressed and the water companies may be struggling keep the pipes pressured up.
One thing you really shouldn't be tending to during dry spells is the lawn. It doesn't look great when the lawn turns brown, but it'll rapidly come back to life when the next rain shower comes.