I was at Kielder last week. Lots of signs up saying no swimming, no unauthorised watersports etc.
Not sure how you get it authorised but at one of the visitor centres saw people getting into wetsuits for a SUP lesson
If I was being generous I’d say they don’t want people getting into trouble etc.
Being realistic/cynical I assume they want you to pay for their lessons or organised sessions.
Compared to this week up at Loch Tay, both ends people launching various SUPs and other floating devices and having fun, and no one gives a stuff and not a sign is sight saying you can’t do it. Much more sensible.