--Care to show where e.g. the LibDems and the Labour parties have demonstrated immigration as a mainstream issue?--

From the Lib·Dem manifesto -

"But since the Tories and Labour abolished exit checks in the 1990s, we have no way of knowing how many illegal immigrants live here.
Liberal Democrats want an immigration system that works. A system that is firm but fair, which plans for the effects of managed legal migration and promotes integration. We believe in the benefits that immigration has brought this country but we do not believe our borders should be a soft touch.
Liberal Democrats would take control of our borders and immediately reintroduce entry and exit checks. Our National Border Force would have the power of arrest."
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From 2010 Labour manifesto--

"Controlled migration brings undoubted benefits to our country but we also recognise people's legitimate concerns about the impact it can have on communities. Net inward migration to Britain as measured by the Office for National Statistics has fallen for the last three years. We are delivering the biggest changes to our immigration, citizenship and border security systems for decades – we are bringing in a new Australian-style points-based immigration system which allows us to be more selective so that only those with the skills that we need to build a stronger economy can come here, and to ensure that as growth returns, we will see rising levels of employment, skills and wages not more immigration."

Satisfied, English Martyr?