No time for a referendum, probably not for a GE either
Any last minute climb-down by the UK - to sign the WA on basically the December terms - would have to be made by the PM, quite possibly by a new PM after May is deposed
The latter would be a new PM who only has the mandate of the HoC, not after a General Election, not even after a Tory leadership contest, other than by MPs at the most.
GEs can be called with a minimum of 17 working days notice, about 3 weeks
(although the Fixed Term Parliament Act makes this more difficult for the PM or Hoc to achieve)
If the govt / HoC see sense at the last moment, even this might take too much time.
Since referenda are not part of the normal UK electoral process, they take much longer to organise than a General Election,
e.g. the question to be posed must be agreed with the Election Commision, there must be official campaigns designated, apending limits agreed etc
We probably have no time to do this before Brexit
and the EU would not want to extend A50 to allow it - and let in Farage & co for another 5 year term -
unless they were certain the end result would be what they want, probably Norway+ or Remain
A referendum or a GE would not give them sufficient confidence to extend A50