While the IMF cannot change the forthcoming UK budget itself, its disapproval would act as a massive signal to global investors. A negative review from the IMF can trigger financial turmoil, leading investors to dump the pound or UK government bonds (gilts), which could dramatically force up the government's borrowing costs, which are already high.
Were the UK government get to the point where it ran out of money and needed a bailout, the IMF could gain direct control of the UK’s budget.
In that (extreme) scenario, the IMF would likely force the UK government to implement strict austerity measures, tax hikes, or spending cuts as a condition for receiving a loan.
Given the above it’s not unreasonable to take heed of the IMF’s position!