Government spending on each person isn't just benefits.
Whilst some state overheads like defence have to be paid for anyway, we employ teachers, doctors, police etc in proportion to the number of people in the country. Number of trains, buses to be built and maintened also goes up in proportion to the population, even wear on the roads.
We're not talking small numbers of people here, it's 1% of the population being added every year. So in 10 years, you need 10% more trains, buses, hospitals, doctors, nurses, teachers, garbage collection, road repair etc. The transport budget alone was £32billiion in 2025. So about £3billion of that was needed because of the extra immigrant population who has come in the last 10 years.
Those costs are easily hidden though, and paid out of different pots, by different people. It isn't the care homes - who want lots of immigrants so they can keep the wages down to NMW - who will be paying the extra 10% transport budget. Or 10% extra police budget. Or all the rest.
Worse, hiding those costs suits the government too. They get to point to nice GDP figures (FDP is a total, so it gets bigger as the population grows - regardless of the cost of that person). They can just ignore the higher state spending ("what can we do?! Don't you want your state services to work??"). And they definitely kick the £7k per year pension liability we're accruing for each immigrant down the road!! State pension is worth £250k, with eligibility built up over 35 years, so each year a person is in the UK costs about £7k in future pension payments - but no one needs to worry about that for a few decades...
Let alone dependants the immigrant is allowed to bring in - without a job, but still using buses and trains, getting credit towards a future pension - so the costs multiply.
It makes absolutely zero sense to bring in immigrants when we have unemployed UK citizens who can be trained to do the jobs. It costs us much, much more. It's just that the cost is hidden, some of it for decades.
It would make much more sense to cut immigration right back and only bring in genuine shortage skills - and I mean post-grad level bio-tech research, not care assistants - and then let the market force wages up so that the work attracts enough UK people. That would still cost much less.
Of course, we should also not sabotage that with excessively high and freely-given benefits. Getting benefits for sitting in the garden is always going to be nicer than working, but shouldn't be an option unless the person genuinely can't work.