You do condition to weather conditions over time. Having been travelling, the first days in a hot, humid climate were difficult, and it began to get easier with a bit of consistency.
The difficulty of the British climate is that you can have 15oC drizzle and 27oC blazing sun a couple of days apart, then back to drizzle again. It's easy to get caught out on the best clothing etc.
I have relatives in a nearby country, their climate is similar but a couple of degrees cooler, and they struggle at 22oC and find that sweltering. It's relative to what they're used to.
Back to the AIBU, this weather isn't sufficient reason to cancel a trip. It can be managed healthily with sun cream (being topped up), hats, light clothing, rest in the shade and sensible levels of hydration.
Over the weekend I've been on a trip with this age group at a theme park. We asked for top ups of tap water at kiosks, thickly applied suncream at the start and reapplied it at lunchtime, rested briefly in the shade between rides and went at a gentle pace. All had a happy, healthy day.