To wear black to a evening dinner- in August?

[hibiscuslightening]

I’ve been invited to an evening function at a very smart London venue in August. I am a bit stuck as to what to wear - I don’t have a big budget for something new either.

however I do have a long black evening dress with sleeves and some beading at the front. AIBU to wear black to summer event?

No you're not being unreasonable to wear black to a summer event.

Why on earth would it be unreasonable?

You can wear any colour you want at time of year. You’re not a 1950s debutante. Seasonal colour rules aren’t a thing any more.