Thank you all so much!
It is Trinity College, and just the once a week formal dinner. I am not getting an award or accolades, so just a mum wanting to experience Oxford with her son. He is in 1st year so maybe he is wrong about the white tie, but he seemed confident.
I am 50 so happy for a structured dress, those shared by @thecatfromneptune look good, thank you.
I had googled but I think I was too specific, Ill do it again. And I forgot about Vinted!!!
Thank you all, I knew you would help xxxx ♡