it's not generally a Yes/No thing, though. For instance, some people might say throw out all immigrants, some might say leave them but don't let any more in, some might say let them in but don't let them claim benefits or bring in family members, some might say welcome tham all. I think each of those views would find support in at least one pollitical party. And if none of them get more than a 50% vote, how do you decide which party takes power and what policy they should proceed with?
Multiply that by the dozens of policies each party offers (and the fact that voters who support a policy from one party may support another policy from a different party)... and you've got a country with 45 million voters and about as many views.