As a EU citizen I'd ask for at least a 2/3 majority to let the UK back into the EU, maybe 3/4. They came, they were always skeptical, they left, they want to come back? Please demonstrate that you made up your mind and won't start thinking about another Brexit in less than 10 years.
Brexit can't just be undone. The UK would have to go through the full accession procedure. This would be much easier for the UK than for countries like Georgia, since the UK system hasn't diverged much, but the special agreements and exceptions the UK had would have be renegotiated from scratch.
Adding a new member state always requires unanimous consent from existing member states, for good and ill.
I was an EU citizen. Then I wasn't. Being an EU citizen means nothing.