London hotel wedding invite

[GenderApostate19]

We’ve been invited to a wedding at The Corinthia hotel in August, haven’t been to a wedding in 20 years, let alone a posh one!
What do people wear to such events?

I own precisely two dresses, luckily floaty summery ones, DH owns a nice suit but is that enough?

We know the Groom and his Dad through DH’s old job and they are lovely people, not quite sure why we’ve been invited though.
It’s a good chance for a summer break in London, we nearly always go in winter.
Do I need a fascinator ? 😱

On the invite, it states that rooms can be reserved at a ‘special’ rate of £600 a night! 😨
I’ve managed to get 3 nights at the Royal horseguards round the corner for £750, with breakfast and a river view!

For an indoor hotel wedding, I’d just wear a elegant summer dress, no jacket and nothing on my head. (Apart from my hair ).
It sounds lovely, enjoy your weekend.

I’m clueless, I really don’t ‘do’ dressing up. I just don’t want us looking like a couple of gate crashers who don’t belong.