Would it still be "Great" Britain if it was just England and Wales?
Your question makes zero sense actually.
"Great Britain" is not a country. It's an island. It has 3 countries on it, England, Wales, and Scotland. It will always be "Great Britain", regardless of any political divisions, unless 1) it sinks into the ocean, 2) some wacky geological process causes it to merge with continental Europe (not likely, the English Channel is actually rather shallow and used to be dry land not that long ago), or 3) people for some reason simply stop calling it "Great Britain" and call it by some other name.
The name of the country you're thinking of is "The United Kingdom of Great Britain and Northern Ireland". Really a rather ridiculous name IMO. It's usually shortened to "United Kingdom". If Scotland leaves, I don't really see why they'd be forced to change the name; it doesn't specify that the UK occupies the entirety of GB, just that it's largely located there, which won't change with Scexit. However, if Northern Ireland also decides to leave this sinking ship, then they really will be forced to change the long-form version of their name. But it'll still be called "UK" for short, even though the union only composes two countries (England and Wales), plus some other territories (Isle of Man, Channel Islands, Gibraltar, etc.).
What'd be really interesting is if Wales also decided to secede. It's unlikely of course, since the graphs I saw showed very strong support for Brexit in Wales, but suppose they all changed their minds and managed to secede. Then there really wouldn't be anything that could properly called "The United Kingdom", since a union of one political entity really doesn't make sense. I suppose they could hang onto it out of nostalgia, or try to argue that the various Shires are united, or that the presence of territories like Isle of Man still make it a "union", but it's pretty weak, but they can call themselves whatever they like. But at that point it'd make more logical sense to just call themselves "England".