Bournemouth Brighton Studlands etc - yep! I'd say you're mad.
Nothing to do with weather or the humidity but everything to do with thinking a crowded beach and hours of traffic is just not a pleasant day out!
But the coast in this weather? No!
Perfectly sensible imo. But then I'm 10 minutes from the beach and being able to swim in the sea and get the cooler sea breeze these last few days has been a life saver!
Although Saturday there was neither sun or breeze and it was like sitting in treacle 
How close are you to the coast?
I would suggest having an early start and getting there for 8/8.30.
Places get busy about 9am onwards. Even quieter beaches will get car parks filling up.
Take snacks, tent, whatever you need and plenty of drinks that have been frozen overnight. They act as a good rehydration tool as well as ice packs!
I would suggest leaving lunchtime or taking picnic and sitting in shade at lunchtime and keeping out if the sun for at least 2 hours.
Ds and I lay in tent, read and play cards!
If staying for the day aim to leave at 4 to avoid the afternoon crowd also leaving at same time.
Sorry if you live near coasts and go all the time and know that those tricks if the trade.
It's just by asking you don't sound like you live near a beach and go that much!