Wow so 20 Celsius or above I thought it was 25 degrees and I thought they could walk but no strenuous excercize? Man they will be cooped up all summer in BC. Any other advice I want him to enjoy the summer but not hurt him obviously.
Bea is especially sensitive to heat, I've noticed - but as long as she can get in and out of her pool (I keep the water clean and cool) she's great -- even in super warm weather. Bo, on the other hand, isn't smart enough to get out of the sun. He's more active than Bea but far more likely to get overheated because he doesn't sit in the pool non-stop like Bea does. I have to monitor him more closely -- even though he's the more athletic of the two. Read up on the signs of overheating in EBs and then watch your pup closely. My first bullie, Mable, could tolerate heat far better than either of these two can. I think each dog is different, for that reason. It's hard to reverse over heating so avoiding it in the first place is your best bet.
Here's how I handle a hot Bullie Summer:
1) Walks in the early morning, when it's cool.
Exercise, discipline and then affection, right?

2) Frequently change water in the water dishes to cold water. They seem to drink more water when I do this - and that's good in hot weather.
3) We don't have AC here, so I position fans in windows in the A.M. so I can get the house as cool as possible before leaving for the day. Then, I contain them - with baby gates - in the coolest room in the house with fans running on the other side of the baby gate -- so there's no danger of them tipping over.
4) I always plan outside activities in summer where there is a reliable, cool water source; lake, river, ocean. ALWAYS. So there's somewhere to cool off. Our favorite park (McCollum) has a stream with a doggie sized swimming hole and cool, forested paths that are perfect for summer walks.
5) If you think it might be too hot to take them out in -- don't take em. Don't risk it.
Some years ago, a fellow I worked got a job back East and had to drive himself, his stuff, his wife and his older EB in AUGUST from Seattle to Connecticut or some dang place. He thought he was prepared; took all the precautions possible right? But somewhere in Nebraska, his dog died in the back seat from over heating. It happened SO FAST too. He was heartbroken. I never want to face that reality and I know you don't either! Err on the side of caution - always - and you and Bongo will be great!