Help! Elegant summery ‘black tie’ dress

[Veeta]

I’m going to Glyndebourne later this year and I’m trying to avoid the last minute dress panic by getting something now. I’ve been once before, and thought that the ladies in more summery dresses looked nicer in the gardens etc. I never wear dresses and always feel awkward in them, particularly very ‘feminine’ dresses, but I’m trying to overcome this.
I’m 40, in reasonable shape, petite (5’2” and size 8). I tend to find I look best in simple tailoring (probs because I’m so short). I’d like to move beyond my usual black or navy blue. It’s black tie, but I found that many women were not in floor length gowns last time, so ideally midi or maxi length. Budget I guess £100-300 range.
If you’re still with me after that essay, thank you. Any help would be greatly appreciated!

This is gorgeous

That Jasper Conran dress is gorgeous. He has lovely silk ones too. Absolutely classic and perfect for a wedding or even a smart event at work — so you'd get plenty of use for it for years. I have two weddings coming up this summer and then an event in Italy in September and this will be fine for all of them. Thank you, @LaChienneDesFromages

This in whichever colour way would suit you best:
https://www.thereformation.com/products/petites-frankie-silk-dress/1311125TEA.html?dwvar_1311125TEA_color=POR&quantity=1

https://www.self-portrait.com/products/magenta-azaelea-midi-dress

Do you like these colours?
https://www.ghost.co.uk/milana-blue-tie-dye-print-satin-dress-d07g026-z75

Do you know what your best colours are?
Lots of range in the suggestions here from hot pink to emerald to cream navy. All gorgeous but some of these colours will elevate some people while not complementing others...

Sorry, had an absolute monster of a workday and then had to batch cook because the ingredients would have walked out the fridge if I’d left them another day. Now tucked up in bed with a tea and looking through the many fantastic suggestions. I’m so grateful for the help, I’d have never have found most of these, and would not have properly considered a good few as I’m so stuck in a rut when it comes to shapes/colours. I think I will order a few of the suggestions here and see what suits - will try a few left-field ones (for me) to see if I’ve been unreasonably excluding eg prom dresses my whole life.

Although I must say, I’m seeing Judas Priest next month and that outfit choice will be much easier!

When I went I found a phase 8 dress that was ballerina length, and actually soft jersey underneath but with an overlay of netting with flowers etc, so comfortable, washes well, packs without creasing, ideal for all formal occasions basically. Don't buy something that isn't comfortable, just no need with modern fabric options

I did consider putting this in my OP but I thought maybe I’m wrong about that too! I can say that neutrals wash me out, which is unfortunate because I’d love to be a lady that can wear beige. I don’t think bright yellow, orange or lime green are my colours either. But I have a lemon yellow top that looks nice on me in the summer. My skin tone is very pale in winter, a bit sallow, but then a pale olive brown in summer, and I have mid/dark brown hair, grey/blue eyes if that helps. 90% of my wardrobe is black or navy, with the remaining 10% mainly jewel colours.

@Hoolihan @Sd352 @minipie @Zebracat i think Karen millen and reformation are going to be taking a chunk of my cash.

@ElleWoods15 that’s gorgeous, I love the material and print. I’m massively jealous of your four nights at Bayreuth as well, my absolute dream.