The UK's temperate, often overcast climate means extreme heat is less of a constant concern than in southern Europe, but it isn't irrelevant — the UK does get genuine heatwave spells, typically in July and August, and UK dogs as a population tend to be less physiologically and behaviourally adapted to sustained heat than dogs in hotter countries, simply from less regular exposure.
On a hot day, the same basic rules apply as anywhere: walk early morning or evening rather than midday, test pavement temperature with your hand before a walk (if it's too hot for your hand for five seconds, it's too hot for paws), and never leave a dog in a parked car, even with windows cracked — UK car interiors heat up just as dangerously fast as anywhere else once the sun is out.
Watch for heavy panting, drooling, lethargy or disorientation as the early signs of heat stress, and move to shade and offer water immediately if you see them. If symptoms don't improve quickly, treat it as a genuine emergency and get to a vet rather than waiting it out.