If we say that there were 3 options on the table, namely "get Brexit done" (Tories + UKIP + Brexit Party), "2nd referendum" (Labour) and "Remain" (the other parties) then I think it would be fair to say that Brexit won a "plurality" of the vote (which is typically called a "relative majority" in the UK):
https://en.wikipedia.org/wiki/Plurality_(voting)
So it's not an absolute majority but a relative majority.
btw, I'd suggest that by the same argument that the SNP (who got 45% of the vote in Scotland) can't genuinely claim to have popular support for a 2nd independence referendum. All the other parties were against it (AFAIK) and there was no third option on the table.