Firstly polygamy is illegal in the UK!!
There is going to be cap on child support at 2 children, but I bet that is 2 children per mother. So a christian family will get child support for only 2 children, but for a certain other religion it will perhaps be for for a football squad!
As for the argument that immigration is needed because of declining birth rates, then surely the logical and most productive solution would be to to provide incentives for native British citizens to have stable marriages and have more kids. That would result in a population that would retain British values, respect and have pride in their country, work and pay taxes. Importing people that hate us and our country, that do not work but live off state handouts, is not going to pay for the future pension and social care costs of the nation. There is no justification for the immigration policy of the last couple of decades, its quite clear what the agenda is, and its pure evil