As somebody said above, maybe a new centre party would emerge. It's hard to see who would have sufficient attraction to lead it to electoral success, but the alternatives of Hard Left or Hard Right might provide an incentive. However, there's a huge amount of latent loyalty to both the main parties, and it would require a gigantic threat to persuade enough of their traditional supporters that it was time to vote for something entirely different.
It's an interesting one. Both parties have the problem that they're trying to be everything to all people, and failing miserably. The traditional supporters of both are pro-leave, so neither will propose not leaving, or postponing it - the fallout in their heartlands would be collosal, and the only guaranteed beneficiary of people deserting either party would be UKIP. None of these people will change to vote for the LibDems - they'll make small gains in the remain voting metropolitan areas at most.