The way I see it is if a party had stood on a “no Brexit” platform in 2017:
For the Tories, it would probably have seen them not gaining 4/5 of UKIP’s vote and possibly losing additional voters on the Brexit side of the party in that direction. Potentially they’d lose other votes from the “Business” type for pissing the economy up the wall for 18 months with no good reason, although that would hinge on Labour or Other looking suitably business friendly.
For Labour I think a chunk of their Leave voters would have gone to UKIP or thrown in the towel and given up on voting entirely.I think they’d also have struggled hugely to get their impressively popular manifesto a hearing over the short campaign and wouldn’t have seen anywhere near as many switchers giving Corbyn a chance because of it.
I also don’t think either party would see huge numbers flocking to them to replace those lost voters because our politics is too tribal - too many people refuse to ever vote Blue or Red - they’re only open to 3rd parties (so long as leaning that way doesn’t allow the enemy to win).
Obviously, all assumptions on my part though.