Hello Games have put up a new ‘support’ page on the No Man’s Sky site, upon which the PC (Steam) release times for the game are confirmed. Sean Murray indicated that it would probably be 6pm British Standard Time in a Reddit session earlier in the week, and that is now official.
Here’s when it’ll be possible to start downloading the game on 12 August: 10am PST (US), 1pm EST (US), 6pm BST (UK), 7pm CEST (Europe). According to the support page linked above, there will be no preloading whatsoever. The download is a reasonable 2.6GB.
As for whether you can actually meet up with any other players in universe (as was stated prior to release), that’s still questionable.