I thought some rays of sun through the drizzling rain constitute a british heat wave, at least at which rate my irish colleagues tend to turn lobster red in the sun led to that assumption
Outdoors, I think something cool, hard to rest upon in a well shaded place is always good. A slab of stone, tile or concrete is well liked. Also places in perpetual shadow where they can dig shallow pits to cool their bellies.
Depending on the setup, an insulated roof does a lot - if it's just one layer it gets hot and radiats heat down, two layers with an air gap and/or insulation in between really make a difference.
My house rabbits have access to the garden but spend most of the hot hours inside, in a cool place on hard vinyl floor, it's an old house without basement so the floors are pretty cool.
I'm not around to wet their ears now and then, which imo does help them too, so they spend the day outside, there are a lot of cool places in the yard, and they can go down to the creek.
