The Lincoln’s Inn Fields/Chancery Lane/Holborn and Temple areas of London have historically been the base for the legal profession.
The Honourable Society of Lincoln’s Inn might work for you - it’s beautiful, has lots of different spaces, and was one of the four ancient Inns of Court.
Or the Law Society at 113 Chancery Lane near Holborn have suitable spaces…
As does the Lincoln Centre, for a more modern vibe https://www.thelincolncentre.co.uk/annual-general-meetings
Or 61 Lincoln’s Inn Fields http://www.61lif.com/. I seem to remember they have nice Indian food as an inhouse catering option.
Sessions House in Clerkenwell used to be a courthouse, although may be slightly too small for your needs: https://sessionshouse.com/uploads/Ennismore_Sessons_House.pdf
Middle Temple and Inner Temple are pretty incredible with a rich legal heritage, might be too flash.
Middle Temple: https://www.middletemplevenue.org.uk/event-spaces
Inner Temple: https://innertemplevenuehire.co.uk/event-spaces/
Hope that helps! Feel free to drop me a message if I can help with anything else, I like finding venues .