"Truth is rarely plain and never simple....." :)
Attempting to take politics out of it, the bald financial picture at the moment is that the country spends more on public services, benefits and other government funded initiatives than it is receiving in tax and other revenue. The country is borrowing significant amounts of money every month and the national debt is increasing very rapidly.
Some (like the Labour party) say that this is not an urgent problem, all countries borrow money, that state funding is needed for investment, to keep people in employment & spending in the shops and that, as the global economic situation improves, the UK's fortunes will be taken along with it.
Others (like your BiL and the coalition) say that high levels of national debt and borrowing are not a responsible way to run an economy, will put off overseas investors, will saddle future generations with the problem of paying it back and that economic growth alone is not enough to fix matters.
If you agree with the 'not an urgent problem' view then cuts aren't necessary because you do nothing & keep spending at the same rate. However, if the economy doesn't improve, you're faced with the same decision and bigger numbers a few years down the track
If you agree with the 'it is an urgent problem' point of view the choices are broadly to increase revenue (raise tax) or reduce outgoings (spending cuts)... or a combination of the two.... until the spending and the revenue are more in line.
Then it comes down to philosophy. A right-leaning government that believes in 'small state' is more likely to cut spending than raise taxes. A left-leaning government that believes in 'big state' is more likely to do the reverse.