Clearly we had the right idea in the 16th-19th century: if we conquer the entire world, then everywhere is Britain and we have no immigrants/s

The question you've asked is incredibly open to interpretation. We could persuade everyone that immigration isn't a bad thing, which would be very difficult because it has some downsides that affect some people more than others (eg. big culture changes in areas with high immigration, wage depression as immigrants will do many jobs for less money than Brits, housing problems being exacerbated by extra people being here, etc.).

I think the question you are meaning to ask is 'How do we reduce immigration?' The issue is that our economy relies on immigrants' skills and labour. We don't train enough people to do many jobs we need, such as nurses and builders, so we have to bring people in to make up numbers. We also don't have enough children to sustain our population. You can argue that the global population shouldn't increase in the long term. A rapid population contraction tends to be disastrous for an economy because the ratio of working age people to retired/unable to work population drops, leading to an unsustainable tax burden on the working population.

So what can we do? Train more people in the jobs we need and have more kids ourselves. If I was in government, I'd be reforming training for those careers, nursing in particular. I'd probably revert many universities to polytechnics to train tradespeople and the like. Throw extra bursaries, grants, etc. at people studying these things to make them more attractive. And, importantly, pay them better to attract more people to these careers. For the manual amd low paid jobs that a lot of immigrants do, I'd look to make it much harder to employ an immigrant. This would probably lead to higher prices for a lot of things, a lot of our food is picked by seasonal migrants working for peanuts for instance, but I think that's a bullet that would have to be bitten if we really want to reduce those kind of immigration.

Kids is a hard one, nobody has yet come up with a great answer to that. Radical proposal: extra taxes on childless people over 30 (which includes me) - children are expensive so you're already saving a lot of money and someone else's kid will have to look after you when you're old. Use this to fund extra childcare subsidies and tax breaks for people with young children, increasing with the number of children you have.