Of the major parties only the Liberal Democrats have consistently and officially voiced strong opposition to brexitBrexit in principalprinciple. Up to the point where the election was called they have been mostly arguing for softening the terms of brexitBrexit. It is fairly unlikely from current polling that the Lib Dems would win an outright majority but if they did there is a good chance that they would see it as a mandate to reverse the process (if that is even possible now) as this seems to be the central platform which they are campaigning on.
The Labour party at least officially was opposed to brexitBrexit in the referendum campaign but their actual campaign sent rather mixed messages and this has certainly been a point of tension between the leadership and parliamentary party. Subsequent to the referendum they seem to have regarded it as a done deal and have tended to support the government on the issue overall eg the leadership supported the vote to trigger article 50.
Also the Labour party is in a bit of trouble at the moment, polling very badly indeed and has a fundamental internal rift and it seems that a pro-brexitBrexit line is likely to play better with their target electorate plus the leadership seems fairly ambivalent on the issue.
Oddly UKIP are a bit of an unknown quantity, given that the conservative party is now firmly on a pro-brexitBrexit course 'independance''independence' is no longer a unique selling point for them and their new leadership doesn't have the same high profile that Farage achieved. Having said that they could potentially take away socially conservative Labour voters.
The SNP are firmly pro-EU but this is tempered by the fact that they might see brexitBrexit as a way to leverage Scottish Independence.
It is also not entirely beyond the realms of possibility that Labour MPs could defect to the Lib Dems en mass or stand as independents on an anti-brexitBrexit platform.
The Lib Dems have little to loose on the issue and a lot to gain as getting the anti-brexitBrexit camp to break party lines and vote for them would be a big win. Indeed this has already happened to some extent. So it seems like their best position is to campaign on a firm brexit scepticBrexit-sceptic platform and stick to it which is basically win/win for them as they either lose and have to do nothing or win and have a mandate to do what they want.