Argh, I should have gotten that one right based on the Principality part of the name, but I focused on Millennium instead and was thinking it was associated with the Millennium Wheel and Millennial Dome.
Millennium Dome got me too! As a non-Brit, I didn't realize it wasn't a stadium (nor did I remember it had been renamed the O2).
I saw the "Principality" part, which gave me pause... but at the end I figured that was a red herring, because: no. The Millennium Dome is definitely in London.
I never heard of the largest lake, but mostly based myself on the '3%' clue : realistically, could only be in Wales or Northern Ireland, cause 3% of Scotland's or England's surface area would be quite *huge* for a lake... (wanted to guess Ireland, chose Wales, i was wrong) (also i'm lousy at distances i have no idea how small/big Wales' surface area is)
But also, yeah it's totally visible on the map, never saw that xD It doesn't help that the question is quite far down, you don't even remember the picture exists.
I missed 3. Predictably, one of them was about sports. I'm actually very surprised by the one about the official flag. Thought that would definitely be England. Interesting.
As with so many things, the Troubles is the reason Northern Ireland doesn't have its own (official) flag. CGP Grey did a great video about this: https://www.youtube.com/watch?v=gaQwC5QbLeQ
I saw the "Principality" part, which gave me pause... but at the end I figured that was a red herring, because: no. The Millennium Dome is definitely in London.
Ah well!