While there are many people and organizations who are adamant about the "necessity" of keeping bunnies indoors, every legitimate concern (hazards like heat, cold, rain, predators, potential for escape, etc. as well as whether they'll be offered enough opportunity for exercise and get enough attention/truly be considered part of the "family") can easily be countered by anyone concerned and educated enough about outdoor bunny safety who also appreciates their bunn(s) enough to spend lots of time out there with them.
You can predator-proof, rain/wind/snow-proof and escape-proof an enclosure, keep them comfortably warm in the winter and appropriately cool in the summer (though at especially extreme temperatures like Alaskan winters or 100-110F+ summer temps, that may necessitate finding a way to either climate control a shed/barn/garage of some sort or to mitigate the "shock" of bringing them indoors temporarily), heat or add ice to their water bottles as needed, build a secure and covered run area or take the time to supervise their exercise in an uncovered pen and love them enough to show it even when that means suffering extreme weather to visit with them. By the same token, being indoors does not inherently guarantee a bunny won't suffer neglect.
In my opinion, the "bunnies belong indoors" mantra isn't applicable to those with outdoor rabbits who do things *right* - it's passed around to discourage all of the people who would likely screw it up
. Also, the issue of people failing at outdoor rabbits is much more prevalent in the US than the UK because your climate is very mild as a general rule compared to what the US tends to experience; our weather doesn't really do "moderation" - it prefers extremes. Ignorance can *easily* be the difference between life and death when winter weather reaches -40F or summer afternoons see highs of 100-120F!
Personally, I DO encourage keeping pet rabbits indoors when possible *if* opinions on that topic have specifically been solicited (ie if someone asks which is better, I generally hedge in favor of indoors unless there's a reason specific to their situation that makes outdoors a better option yet if someone with outdoor bunnies doesn't ask for others' thoughts on the matter, I don't try to push an opinion on them)... but I'm adamantly opposed to stating it as an absolute necessity (as though living outdoors automatically guarantees "second rate" care). While some people keep their rabbits outside because they whole-heartedly believe it's in their best interests, many people would honestly prefer to have them in the house but feel they can't or shouldn't due to certain circumstances (among the possibilities are allergies, lack of space, safety concerns regarding other household pets or their parent(s)/significant other/roommate insisting the bunn(s) belong outdoors).
Those who aren't in a position to feel they can freely choose one way or the other without opposition or consequence end up feeling guilty because of the stigma that's been created around keeping pet rabbits outside. They often feel they're perceived as not being as good an owner as those who have "house" rabbits (or worse yet, actually believe that)... when really, the location of your rabbits has NO bearing on what quality of bunny-slave you are. As long as you ensure that all of their needs and some of their wants are met (the philosophy my parents raised me by), then you're a wonderful owner.
Respect the many ways in which domesticated rabbits' needs differ from their wild counterparts/make the appropriate accommodations and outdoor "pet" rabbits can live perfectly healthy and happy lives which are no more or less fulfilled than an equally well-cared for house rabbit.
:soapbox I'll quit rambling in an ancient thread now, lol... I just hate to see a great owner feeling guilty for no good reason!