There isn't a pie that is national wealth to be divided up and therefore more people = poorer people/ fewer services.
The US population is 5 times the UK population. Does that mean 5 times the people competing for the same number of GP appts? Of course not: with 5 times the people they have many times the number of doctors.
The quality of immigration obviously matters. Back when most of our immigrants were from the EU they contributed more tax than the average UK citizen, were more highly qualified in skills shortage areas, most likely to start their own businesses and create jobs employing others, and used social housing and public services less than the average UK citizen. Therefore, they raised the average standard of living for the average UK citizen by their presence here and their contribution. But people thought this was a terrible thing, apparently.
It is absolutely Government policy to determine what immigration we allow and from where, whether that immigration is beneficial to UK citizens, and to ensure that sufficient additional tax revenue generated by the population growing from immigration is directed to expanding services to accommodate the growing population (while still making a tidy profit on top from the surplus tax revenues, if they have an appropriate immigration policy and are accepting the people we need).
Obviously Government policy affects every area of your life. The public and state sector ponzi schemes are draining tax revenues that need to be spent on education, infrastructure and supporting key growth sectors so we have productivity increases and rising living standards.
Not reforming the NHS into a functional system like most other European models means we have some of the lowest health outcomes in the developed world for not much less as a share of GDP.
Selling off water companies has meant they have extracted billions on dividends without any sustainable business plan to fund the infrastructure improvements to sewers and resevoirs to serve the current population and lack of Government regulation and enforcement means they continue to pump sewage into our rivers and seas.
Selling off utilities means we now have Canadian, French and other nationalised companies making huge profits on us paying the most expensive energy prices in Europe while they can subsidise cheaper energy for their own citizens and businesses. This also means the UK is less attractive for foreign investment.
We have an enormous trade deficit as a result of Government policy (quadrupled by Brexit) meaning we are extremely reliant on external investment to stay afloat.
We have no long-term industrial policy, healthcare or pensions policy, energy or food security policy. Brexit has left a £50bn per year hole in HMRC revenue, rising annually.
The education system is a shambles with no effective regulator to enforce the law and is not equipping students with the skills required in the modern economy. Young people are saddled with debt when higher education is free in most comparable countries.
Childcare is more expensive here than in the vast majority of countries. Germany, France etc charge a couple of hundred euros per month whereas in the UK people have to pay thousands. This harm productivity and economic participation.
Our tax system includes numerous cliff edges which are preventing economic growth and productivity increases which are the only ways to raise living standards. This is well documented in rigorous economic studies yet no Government has removed them. Consequently people either work part time, retire early or the highly skilled emmigrate, against lowering overall living standards and tax revenue.
All of these are Government policy choices. All of them are proved to be negative for UK living standards since there is decades of data from other countries proving that their models work far better, which our Governments could choose to emulate.
I find it insane that any adult could think Government policy doesn't affect their life. I agree that there's little worthwhile difference between Labour and the Conservatives because both have (different) but ineffectual policy objectives that will not improve things at all. But that doesn't mean policy is not important and cannot make an enormous different to people's lives if we actually had competent people in charge.