Thanks for posting. Have you considered ExCeL London’s Platinum Suite? The website mentions from 400 delegates but having worked on a few events there (including the British Motor Show), I don’t think 250 would be lost and there’s room for the growth you mentioned. The room can easily be reconfigured for dinner. Plus you can get vehicles into that space via a lift if you want to showcase your latest models as part of the event.
ExCeL is right next to London City Airport, has numerous hotels on site and nearby at varying price points: https://www.excel.london/visitor/stay and over 3000 parking spaces.
Hope that’s helpful. Let us know how you get on.