I think actually you've shot yourself in the foot here though.
The title is "To think it's a bit late of the school to stop Ds".
But the issue seems to be that his behaviour has deteriorated recently (probably down to SATS, but it could be other things like knowing there's a change of school etc). So they were happy to take him before his behaviour deteriorated, which was great.
However you're assuming that his behaviour will improve. You can't guarantee it will, nor can you say how much it will improve after sats.
But from this, you're asking them to make a decision later. You obviously think that the decision later will be that he'll be fine. But what if he doesn't improve afterwards? He'd be getting an even later ban.
It may well be that if his behaviour does change like a switch after SATS they may be happy to come back to you and reconsider. I think that would be better than at the very last minute saying he can't go.
On the basis that they were happy to take him until his behaviour deteriorated I think they are prepared to make adjustments, so you'd be unwise to go in with that as your main argument.
Knowing my children's residentials in year 6, they're very activity based and fast moving. If he takes 1 member of staff half an hour to settle, he'll have missed the safety briefing and then not be able to do the activity.
The activities they were doing are ones where you can't risk children's safety. Abseilling, kayaking, skiing etc. I personally would have liked to see some activities that didn't just cater for the outdoor sporty groups, but they don't do those. In those, one silly moment, or a missed safety briefing, panic and turning back at the wrong time, can certainly cause serious injury.
Go and talk to the school, but do it on a "can we make it work for him to go". Maybe they'd be happy for you to bring him for the day (that was done at dc's school with a child who hasn't safe at night) or for you to go, or if he can show XYZ.
If you go in with a "you can't do this, it's not fair" I'll think you'll find that they stick firm.