As with everything it's a question of political will.
Of course we can 'afford' to have decent pensions for all, of course we can have a decent living wage, of course we can afford to have free higher education, a free NHS etc etc. Indeed it makes sense as when poorer people get more money they spend it, boosting the economy.( Btw China though having a much lower income per capita allows most women to retire at 55. Only managers need to work to 60.)
What we can't afford is allowing the very rich to off shore their wealth, or major companies avoiding tax, or privatising our water, electricity, gas, transport etc so we pay more so the billionaires can offshore the profits from these industries instead of investing in our Country's needs.
We have a 'fiat' currency. Our currency is not based on the amount of eg gold we have; it is based on us ( and others) 'trusting', that the Government will act sensibly ( fiat means trust). National banks create, control and issue the money supply. If the Government needs more money to invest they issue more gilts ie they create money and lend it out.
We and foreigners eg look at our economy and decide whether we/they think we have a more or less viable, trustworthy, economy and will be able to continue to pay the interest and return the original loan.( We have never failed to this btw.) If not they/we want a higher interest rate on gilts for lending us money because of the higher risk they might lose their money. If they think we have a sensible economy then they/we are prepared to get a lower return in return for the security.
Selling off our industries, our services, etc to private investors ( eg by Mrs Thatcher but also by later Conservative Governments) has damaged our economy by taking money out of our country ( off shoring by the rich and foreigners) instead of investing here. If we can reinvest in our country, nationalise industries ( Steel, water, transport etc), stop the privatisation of the NHS to US interests etc. costs will come down and we will be able to boost the economy.