> Which is... quite weird, really, because the former opt-outs were mostly fairly minor.

Schengen and the Euro were hardly minor:

* https://en.wikipedia.org/wiki/United_Kingdom_opt-outs_from_E...

There were only four (now three) countries in the EU that got any opt-outs at all, and the UK the most opted-out items (4):

* https://en.wikipedia.org/wiki/Opt-outs_in_the_European_Union