I think that the boats thing stirs up ideas that migration is out of control, that the government is unable or unwilling to get a grip on the situation, that the system (even if they don't know what the system is, or even if there is a system) is being abused and somehow cheated. That's (IMHO) why it's so easy to get people riled up on irregular migration.
I'm not sure if they end that route that they would need to out-anti-immigrant the anti-immigrants any further, but in the current climate they will need to be able to make the case that the country can decide who comes in, and that migration is to the benefit of everyone, migrant or not.
Again, it doesn't really matter if it's an actual problem, it is an important enough perceived problem that they need to be able to show they have a grip on it and are running the show in the interests of the average Brit on the street.
Then to really put the issue to bed, they'll need to do something about the failing services and general feeling of decline in the UK. As I said in response to a sister comment - you don't get many nazis when people feel their lives are going well. It's not so concerning if some out group is getting a slice of the cake if you feel you're getting yours too. It's when your slice seems to get a little smaller every day that you start looking for scapegoats.
Of course the other question is - will they actually lose the left wing vote? Or would they win it back?
Opinion polls in UK politics (from what I've heard on the radio) put the politics of 'Reform' voters left of centre - they're keen on renationalising rail, water and electricity for a start. All solid left-wing ideas outside of immigration policy, that you'd usually expect to hear from Labour supporters.