My advice would be very different. It works perfectly for me, I'm short 5'3 and apple shaped, weight fluctuates a lot any this formulae works along the scale.
I understand flowered tea gowns or frocks are ideal for afternoon team and look lovely. They also tend to look particularly lovely on 'pear' shapes and do little for apples.
I'd start with a pair of these
M&S high waisted leather look
Leggings, which are very good quality and ridiculously cheap but really thick and hold you in. Very adaptable to dress up or down. I know they look good because I've worn them to 'show' to every fashionable person I know and they have all already got a pair or told me they bought a pair on the way home.
Wear them with casual flats or clean trainers for travel and change into heeled ankle boots when you get there.
Carry the boots in a 'decent' bag and no one will think twice is you disappear into the 'ladies powder room' on arrival. There will always be one away from the room where your afternoon tea will be served so you can make an entrance once all sorted
Wear with what ever top suits your face shape perfectly, for me that's quite a deep v neck and a bit of extra fabric or wear with a full, long scarf to skim over my tummy. Team with a jacket for me that's a 'rock chick' style zipped leather jacket which is quite short at the back to show off how trim my hips and legs are.
Necklace and earrings, touch of natural makeup and even my mother would be impressed.
You will look great, be very comfortable, look absolutely 'with it' but absolutely not try too hard. You will also be prepared for your other event and may find other events appear for which you now have the perfect outfit.
I am very happy to wear the same thing everywhere a bit like a uniform. Different tops, scarves but no one really notices the details, so long as the overall effect is so great for you and for now
P.s. if you do think about getting the leggings, M&S's click and collect service is great. You order all the sizes and colours you want to try on to a particular store, try them all on and give back any you don't want on the stop for a refund. Really easy.