It's not to them. 95% of it is spent on all the obvious things - NHS, social support, education, transport, defence, etc. 5% of it is spent on interest servicing the £1.8t.
That £800b of annual spending is paid for by £740b of tax/rates income and £60b of extra state borrowing per annum...
That's cash accounting.
Accounts and tax returns are normally based on the accruals principle, that is the value of work done or goods despatched in a period = turnover/income.
The date sales invoices are issued or monies received, isn't the determinant of business income.