I fully agree with the previous posters that it is largely a combination of high house prices (couples where financially possible prefer the security of their own home when starting a family) along with the high cost of childcare which encourages parents to have fewer children.
That said I personally don't see falling birth rates or equally stopping as much immigration as possible (legal or otherwise) resulting in a shrinking UK population as necessarily a bad thing.
If in terms of the environment the single most effective measure is population control and in terms of the long term wealth of the economy a smaller population would equate to a higher GDP per head. In terms of quality of life then a smaller population is better.
To be clear I am absolutely not advocating some lemming type mass cull of the population or some Chinese one child policy but rather a gradual reduction through voluntary lower birth rates and reducing immigration.
Sure there is the argument about growing the numbers of workers to support the ageing population but this is effectively promoting a pyramid scheme where you have to keep increasing the workforce to keep paying for the pensioners and then more people for when that workforce retires and so on. It also assumes everyone in the workforce is actually in employment and a net contributor of tax revenue (ie put in more than they take out).
This is is not sustainable and quality of life, infrastructure, environmentally or economy I don't think the country could handle another 40% increase from 75 million to 100 million (UK population increased from 50 million in 1955 to 70 million in 2024)