It's never going to be lawful in the U.K. to have some sort of blanket "we don't take you if you are from x country" and nor will it be to have an unspoken rule that the UK doesn't accept people for visa applications from particular countries.
What would be, assuming you wanted to reduce the immigration rate is to change the amount required by means of a fee to apply to charge more, to insist on a declared amount in funds to meet medical costs by way of insurance, changing the point at which employers can seek visas so that lower paid jobs are excluded, dependents cannot come, age limits are imposed so you must be under 40, educational requirements are to UK standards, no recourse to benefits, and no route to citizenship.
All of these things are possible. They would make employers pretty unhappy. But they would also make the UK very unattractive for someone who had very little but wanted to work their way up.
The other half of the debate which is being discussed and I don't think that is a coincidence, is welfare and an ageing society. The migrant is younger, fitter, probably uses public services less than an ageing British person. The amount of investment this country would need to turn around its issue with sickness and long term economic activity hasn't been done. It's cheaper to get in people from overseas. And if you are a laissez faire Conservative who is already pretty well off then you will be happy with that.
And underlining that is something even worse; that most working Brits do not contribute tax unless they are paying higher rate tax. That is 10 percent of our working population.
I'm not pro immigration or against. But there does need to be a real discussion of the economic reality of why we have it, what our own problems are in terms of productivity and this feeble tax take we have, and then what can be actually afforded for public services. I hear people saying they want better services and living standards; good idea but it all rests on the people we have working harder, and more of them, and paying more tax. That is what the boomer generation did. They paid far more tax. They worked longer hours. They had lesser public services and they had housing subsidized by the state via social housing or tax relief for mortgages.