Brexit/Bremain is one factor, but Bremain/Soft Brexit/Somewhat hard Brexit/Hard Brexit/Ultra hard Brexit are not. This is hands down the single most important issue UK is currently facing, and the issue that will determine the future of the UK, maybe even until the end of this century.
That is not to say any party that wishes to capitalize on it should make Brexit the single point issue. No, it should make it the most important issue, expose as many failings in the approach to Brexit as possible, denounce failings of all Brexiteers and claim they're incapable of handling the job and claim they could do better - Brexit or no. This isn't hard to do: open a debate on what kind of Brexit does UK want, have an open parliament session running for a few weeks, followed by a secret vote (if British system allows for that) on what kind of Brexit is desired, then execute that.
EU is not keen on punishing Britain, who is an important trade partner and ally, but the British approach to it is giving EU a hard choice: punish UK or risk falling apart, triggering much worse results than any punishment of UK could ever bring. In light of that, an Ultra-hard Brexit with WTO terms and more is a distinct possibility. Seeing that having your hands tied down a bit by a parliamentary vote is more of a lever than a hindrance.
McHrozni