So Saturday is from 5pm at the sushi place? Will there be some kind of meet-up earlier in the day before heading onto the sushi place or is the meet just meant to be that (possibly somewhere afterwards I'd imagine)?
It would seem a shame, especially to those travelling from futher away, that the meet-up would start late afternoon/early evening. I for one wouldn't mind meeting up somewhere else earlier, like the London Eye or Trafalgar Square or something if other people fancy it, even if it's not part of the official meet (I know you're trying to keep this low-key so I'd understand if you don't want the fuss). Then we could grab a couple of pints somewhere and then join the rest at the sushi bar nearer the time (I might grab a burger somewhere on route as I'm allergic to fish).