Admittedly, I don’t know much about Brexit, but from what I have been exposed to, it seems like a decisively economical and political impairment that made travel and business with the rest of Europe more difficult and costly. Since it is so highly criticized as a terrible move, why doesn’t the UK just rejoin the EU?
I think they do, but probably not guaranteed everywhere. Denmark might not, but I don’t know for sure. I know Czechia does close to the borders, but I haven’t been to the other countries myself to know.
It is up to the shops whether they want to accept euro or not (or any other currency), but the official currency is kroner. I know that some supermarkets (Netto) used to gladly take euro in exchange for a horrible exchange rate