Is Reform UK's migrant detention centres plan legal?

I suspect the policy isn't intended to actually build detention centres. It will be used to generate lots of daily mail (et al) headlines about green councils not wanting migrants. It also allows them to blame the greens if there's a story about bad conditions or overcrowding at existing detention centres, "we'd love to build new centres but these meanie greens that claim to love immigrants keep stopping us". It's actually kinda genius in an evil way.

planning law, and immigration laws which allow government to build immigration centres do not give any authority to decide where to detain people based on the voting patterns of citizens nearby.

They could use a proxy, find something that green councils have the most of (allotments? Universities?) and claim to be using that to site them.

Reform could try to legislate its way around the courts – a Commons majority could in theory pass a new act giving politicians the power to site detention centres by voting pattern.

But in reality, this would face a “hell of a ride through parliament”, said King, “especially the House of Lords, where there would be few (or no) Reform peers to defend it.”

The lords can only delay legislation, they can't stop it.

Wrong question.

The question is how to stop Reform before they take over. Once they're in, legality won't matter.

Not only would it be illegal the threat itself could be considered voter manipulation. Threatening people to vote a certain way by saying your political opponents will be punished is clearly manipulation.