Make sure your vet knows rabbits. A good way to find a rabbit-knowledgeable vet is by using the House Rabbit Society's links or in the UK, the Rabbit Welfare Association. We almost lost a rabbit due to a naive vet.
Buns need their vaccines. In the UK, rabbits should be vaccinated for haemmorahgic fever (VHD) as well as Myxomatosis. Always have new buns vaccinated before being in any contact with those already living with you. Now you can get a combination vaccine of both.
They should be spayed/neutered as this will ensure they do not breed, calm their temperament, and they will live longer. It will also help prevent spraying.
Rabbits will suffer in the heat and can also die of cold exposure. Make sure outdoor rabbits have a warm house in the winter with plenty of hay and straw for bedding, and insulating for winter outside the house is a good idea. (we use sheepskin in one house; bubble wrap on the roofs but not inside where they can chew it) In hot weather they need air circulation, plenty of shade, and plenty of fresh water. We also freeze water in large plastic bottles and put one or two in the runs on extra hot days when it is above 24 c. (we use rinsed-out empty milk and juice bottles) For shade, we bought greenhouse shade screen and put over the tops, southern and western sides of the runs. Also we planted bun-safe trees near the runs, like willow and apple, for natural shade. It is also useful during downpours of rain because it filters it so the buns can be outside of their houses during rainstorms and not get drenched, if they like. House rabbits can also get heat stroke. We have fans we use in summer (with the cords safely behind a book case or a bun proof fence) and for Burgess, who has a lot of fur, she has a "cool stone" which can be purchased at a pet shop. I use a large kitchen floor tile, which is bigger. Any stone will not do, some get hotter with heat. Limestone, marble, granite, and man-made ceramic stone will stay cooler. It needs to be big enough the bun can stretch out on it, legs and feet and all and put it in a comfy location on the floor out of the sunlight.
In case of heat stroke (from the House Rabbit Society): DO NOT submerge a rabbit in cold water. This could place the rabbit in shock.
Dampen the ears and body with cool water. GET HIM TO YOUR VETERINARIAN OR EMERGENCY CLINIC IMMEDIATELY. DO NOT WAIT!
House Rabbit Society rabbit health link
make sure your buns have plenty of fresh water every day
we use greenhouse shade screen on the tops, southern and western sides of our runs; this type of screen filters out 60% of incoming sunlight in summer; on very hot days (above 24c we use white curtains, sheets and other cloth for shade and create a tent city in heat waves (see photos at bottom of page).
sheets and curtains over the runs provides shade on long hot summer days when it's over 24c, held secure with Bungee cords