Thanks for all the great suggestions, we've got a plan for the rest of the week so I'm feeling better.
We found a lovely, shaded walk along a low river this afternoon which was perfect. He was still panting by the end of it but he was ok.
Tomorrow DH is taking DC to a theme park, I'll walk dog early and have a day to myself at the cottage (bliss!)
Wednesday we're going to the beach super early for a few hours then back for the mid day heat. We're going to leave the dog in the cottage while we go for ice creams in the afternoon. He should just sleep but we're going to set up our phones on facetime to keep an eye on him and we won't be away.
Thursday we'll do similar and Friday DH and kids are going biking in a forest and I'll walk the dog and come back if it's too hot.
Hopefully that's enough to keep everyone happy. We're going abroad in October and will be leaving the dog with family so I'm not too bothered if the kids find this a bit boring.
It is difficult having a reactive dog, that plus the heat really does restrict things although I'm seeing how improved he his this year. He's still only 3 so hopefully we can master a pub garden at some point!
He's a cross bread for anyone who asked (I think someone did!). Mix of spaniel and terrier.