Hi everyone - I know the OP's query is now out of date, but for everyone interested, here's a site about heatstroke in dogs, run by academics who do research on heatstroke in dogs: https://heatstroke.dog
The tl/dr bit is that the single biggest cause of heatstroke (which does cause death very regularly in the UK, because dogs here aren't acclimatised to heat, so this isn't overblown, and evidence from abroad is irrelevant) is exertion, meaning walks. One of the other major triggers is travelling in a hot car. Some dogs are far more vulnerable than others: flat faced dogs and Golden Retrievers, for example. Also older dogs, sick dogs, double-coated breeds, puppies, and adolescents. The reason we're okay and they're not is because we have entirely different heat loss mechanisms; and it takes about 60 days to acclimatise, so heat-related deaths are much more common here as we generally don't have 60 days of warm weather in a row.
There's also a lot of bad advice out there. If your dog starts overheating, don't faff about wetting their paws or draping them in towels; soak them in water that's colder than the dog, then get them to a vet in a cold, air-conditioned car.