I am just a little bit smaller than you and a few years older.
I think a dollop of confidence is the main thing (this couple sound like utter cunts), can you make sure you have hair, make up, perfume, bag and jewellery that you like, and shaped eyebrows and manicured nails and all that? On the plus side, at least if you are staying you can drink. Take some snacks and maybe gin in a tin so you can have a quiet boost if you need it.
Being appropriately dressed massively helps though, I recently did a wardrobe overhaul and did get a bit of help from a stylist. Semi skinny or well fitted slim/straight dark trousers and slightly fitted shirts and tops in brighter colours, not too thin material and just long enough to cover my stomach, but not so long they dwarf my legs are what seem to work best. (in am a 22/25 WIP)
what I have found work so far
M and S Eva jeans, M and S straight cords.
Seasalt Larissa shirts in the day (avoiding the twee patterns), and various from Mango, River Island, m and S, Vera moda, Wallis
Mostly wallis monsoon and mango for evening tops
Seasalt for jumpers. M and as for coats.
All of these are quite generous in sizing so 24 should fit you well, I don’t think you’ll need the 26.
It’s not super exciting but I am a good bit more pulled together than I was.
The weather is still shitty so take walking boots rather than trainers and a good waterproof coat with a hood and gloves, change for the evening and keep in mind you might get wet on the walk, and will anyway want to look nice at breakfast so you will need another outfit for that. If this is the country house weekend it sounds like, bring a new outfit for each activity, partly because mud, and partly because it’s just what people do and so it will give you confidence.
If you feel the cold, big country houses can be chilly so make sure you have something warm for the evening - although you will probably want to flaunt your terrific cleavage so maybe a shawl you can drape.
If you’d like a stylist name do PM.