Glyndebourne

If you do a google-image search for “Glyndebourne audiences” you will get a very good idea of what people wear.

And this slightly snooty rticle may help!
http://www.townandcountrymag.co.uk/style/fashion/how-to-dress-for-glyndebourne/

And here’s a really useful Mumsnet thread:

https://www.mumsnet.com/Talk/style_and_beauty/184812-what-do-i-wear-to-glyndebourne

Glyndebourne’s own website adds this:

Women may absolutely wear palazzo or evening trousers, but if you’re
opting for a matching jacket or blazer, avoid anything too reminiscent
of office wear.

It’s seen as quite a posh event and of course for many people it’s an outdoors/indoors event as it’s traditional to picnic in the grounds. I think your proposed outfit, while a good base, may not quite be glam enough. Maybe a beaded top of some sort or a really beautiful pashmina or shawl?

Surprisingly, summer festivals are more formal than the regular season. I heard that Glyndebourne was a "No Tux, No Go" zone. I did go to Bayreuth a few years ago and tuxes (for men of course) was over 60% I'd estimate. I got a bow tie so I wouldn't look too out of place. Only the confident dilettantes went without ties. Girls from Japan, I talked to one at our hotel, said dressing in a kimono was a 2 hour operation.

Thanks for your help! Those are very helpful sites.