Hello,
Your last two edits have broken the Jersey National Park so that it is no longer a valid multipolygon.  I suspect that the easiest way to fix it is if I roll back those two edits, you change again what needs to be changed, but then (and this is the important bit) you then check it with JOSM's validator to make sure that it's still a valid multipolygon.
Let me know if you'd like any help with any of that,
Best Regards,
Andy
