Another explanation is that they were 100% anti-immigration but also 200% incompetent at implementing their own goals.

Rather than being honest with people about the pros (of which there are many) and cons (of which there are some) of migration, the conservative government increased migration while telling people they were reducing it. That is malice, rather than incompetence. And now, here we are, facing the very really real possibility of loathsome grifter Farage as Prime Minister.

Same with the Republicans in the US and the deficit, "small" government, etc.

US conservatives had the same issue before this year. They'd regularly vote against stronger immigration policies, because it turns out rich donors like under-paying non-citizens.