Perhaps I’ve been gone from the U.K. too long! That is totally, 100% what every other mother is wearing during the summer months where I am. It’s an ordinary, floral, lightweight, floaty dress - one garment (easy to put on), cool/breezy, basic colours. Normally worn with a straw hat / sunglasses / flip flops or flat leather sandals / crossbody bag. It’s either this (like a uniform) or workout gear or work clothes. I don’t recognize the “Primark leggings and oversized T shirt” thing - so, wearing what one would wear in normal life to do normal things like take your kids to school and run your errands to a wedding just wouldn’t happen here.
By structured and formal I meant exactly what Kate Middleton wore at the weekend to whomever’s wedding: pleats, defined shoulders and waist, neat collar, high heels, appropriately accessorized. Nobody would wear THAT in daily life!