No, it's patent-law, not copyright law. Interestingly while the US has Fair Use (which is an open-ended legal defence), I don't think any countries in the EU (or "Western Europe" if Brexit happens) have anything beyond the closed and enumerated "Fair Dealing" defence).
That was true for a long time, but to quote the Government of the Netherlands (the closest thing I can find to an official source) [0]:
>The United Kingdom (UK) left the European Union (EU) on 31 January 2020. A transition period is now in place until 31 December 2020.
As far as I can tell from my distant vantage point on the west coast of North America, there is very little doubt left. Brexit has already happened and most of the fallout will be settled by the end of this month. I supposed you could say that it's still in a constant state of happening, but it's well past the point of "not happening" as far as I can tell.
We'll see what happens. I mean, it's possible that they actually end up going through with a complete exit on Dec 31. My guess, however, is that there will be ongoing negotiations for a long time to come. The UK is likely to be out but only in the sense that they don't now get to vote on EU resolutions. They'll likely make negotiations with the EU which end up putting them right back where they were (regulation wise).
That's my expectation at least. They may feel the full pain of a full exit on Dec 31, I doubt they'll want to continue in that state for very long.
There's also the possibility of yet another extension on the transition period.
The UK ceased being in the EU in January, and transitioned into a temporary EEA membership, subject to most EU law. That comes to an end in a few days; at that point what happens next depends on the outcome of negotiations. There's certainly a world in which EU patent rules continue to apply to the UK; in fact, that would probably have been the default assumption until the British government went off the deep end.
So, if you use 'Brexit' to mean purely the legalistic act of leaving the EU, then yes, it's happened. However, effectively none of the _effects_ of Brexit have happened yet.
I think you're thinking of copyright? Where reverse engineering is allowed for interoperability.