tommytailwind.
Can’t speak for UK/NL but had plenty of experience of UK v another European country and I’m in agreement with your sentiments.
I’d certainly be very surprised if UK HMRC allowed an opt out on NI for somebody operating out of a UK base - it certainly wasn’t the case when I was working but I guess things may have changed.
Ultimately you are right that in order to avoid jeopardy input is needed from professional tax advisors, initially at least.