That’s an irrelevant point.
There was never a referendum in the U.K. to join the E.U.
It is therefore by default illegitimate.
The UK had a referendum in 1975.
There's nothing in the British constitution which requires there to be a referendum before decisions of this kind are taken. In fact referendums have no constitutional status - they only happen if Parliament decides it would be a good idea to have one (usually, as in this case, to resolve an internal debate in the governing party.)