I think UK had some serious negotiating leeway for simply being the UK. EU countries really want the UK inside the Union because it makes for a better union: it's a G8 country, a nuclear power and permanent member in the Security council, an economic behemoth with a large international clout, post-colonial ties and strong industrial and technological heritage. It has a massive internal market and rich consumers.
So it was always accepted that the UK can get more concessions than the average country, simply because they bring more to the table. But there's a limit to that, not even the UK can get what Brexiteers promised: unfettered access to the internal market coupled with full sovereignty to set any tariffs and enter any trade deals. That would effectively be the end of the EU.
So it was always accepted that the UK can get more concessions than the average country, simply because they bring more to the table. But there's a limit to that, not even the UK can get what Brexiteers promised: unfettered access to the internal market coupled with full sovereignty to set any tariffs and enter any trade deals. That would effectively be the end of the EU.