I saw that red dress in the flesh in M&S & thought it looked lovely on the hanger. I was surprised by how many clothes I actually liked in our local not very big Marks, but only one of the 8 things I tried fitted. I do have difficulty finding dresses to fit me, as I am flat chested with broad shoulders. What I really did like was this dress:
www.marksandspencer.com/bubble-textured-skater-dress/p/p22297175?colour=PalePink
As I am blonde I tried it on in black and it was gorgeous. It so nearly fitted but my shoulders were too broad. It was amazingly well made with expensive looking fabric. If you suit shell pink try it on as it looks like a good Ascot dress to me (though take a warm wrap as it's thin), and should go with a lot of hats. If I were you I'd consider hiring a hat. I have hired from somewhere that is an association of farmers' wives that hire out fabulous hats for weddings and Ascot, and they adapt hats by re-trimming to match your outfit. I'll have a rummage and post a link.