Exit polls are held back not for trading purposes, but so that people go to the polls and vote. If people think they know the result they won't go through the hassle of voting.
If they didn't hold back exit polls (for the reason you've correctly stated) paying for priority access to polls would be a whole lot less useful for trading purposes anyway because instead of the big poll reveal at the close of voting all kinds of [partial] poll results would be accessible during the day. But as you've said all this exit polling data available would affect voter behaviour, especially in elections like the UK's with plenty of "tactical voting" going on
If a specific 800 people had changed their mind the tories wouldn't have been able to form a government even with DUP support
However the exit poll in 2017 was pretty much dead on
'Not permitted to be released, which they then used to trade'