> It so happens that the UK population would have started to decrease naturally I believe around 2024 without immigration

I think it would've been much earlier than that. Cumulative net migration since 2000 is around 8.5 million people. So the UK's current population would be radically lower today without immigration.

https://www.statista.com/statistics/283287/net-migration-fig... (annoyingly the per year figures need to be summed up manually on both this and ONS reports)