uk was never a real member of eu, it entered the union with a lot of less strings attached, the only country to get such treatement.

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

The UK was not unique in having “opt-outs” from the EU-other countries have them too:

Denmark has a formal opt-out from the Euro. Several other states (e.g. Sweden) have de facto opt-outs from the Euro - unlike Denmark, EU treaties say they have to join, but their governments have decided to ignore that legal obligation - without consequence, since the treaties failed to provide any enforcement mechanism for it

Denmark and Ireland both have opt-outs from security and justice policies. Ireland’s opt-out permits it to opt-in on a case-by-case basis, Denmark’s doesn’t. Until it was abolished in a 2022 referendum, Denmark also had an opt-out from the EU’s military policy

Poland has a partial opt-out from the EU’s human rights charter

Ireland has an opt-out from Schengen (abolition of border controls). Romania, Bulgaria and Cyprus are parties to Schengen, but haven’t yet implemented it

The UK's opt outs were to rules brought in during its membership, not prior to it's accession to the (then) EEC.

Really?

https://en.m.wikipedia.org/wiki/Opt-outs_in_the_European_Uni...

Your linked page says that Britain had four opt-outs before they left the EU, more than any other country. At some point it even has five opt-outs.

Not sure if they had opt-outs immediately when they joined the union (as the grandparent claims). But they were surely the member for which most exceptions were made.

Some non-Brits were actually happy when Britain left the EU, since it removed a member that frequently blocked or slowed down integration. Not sure whether this is true though.