Of course it hasn't! To understand why, see below.
The 4 biggest revenue earners are Income Tax, National Insurance, Corporation Tax and VAT. Together they make up about three-quarters of the total. Manifesto and other commitments mean that these are "out of bounds" for further changes to the headline rates, and hence for raising significant amounts of additional revenue.
And that's why things that cost £1.6 billion now have to be scrutinised and justified.
That's in the past. The future is for continuing deterioration because of lack of economic growth and an aging and more sickly population.
As a result, there are lots of things that the government used to be able to afford, but can't afford any more.
Taxes have been lower in past times, and Government-provided services better. Despite tax on individuals being higher than it has been for decades, there is a pervading sense that nothing really works properly any more. And if the Governent slashed Gift Aid, it would quite likely lead to charities being able to do less,and the Government having to take up the slack in some of those areas - which might well cost them more than just leaving GA alone.
As always, the Government can afford to spend money on things they want to - overseas aid, military aid to Ukraine, vanity projects like HS2 (which everyone knows before work starts will go way over-budget, because that always happens), and meanwhile national institutions become more and more bloated with jobs which don't need to exist. e.g. EDI departments didn't exist a decade ago - it was just part of HR - but it's become a ballooning industry now.
And as regards economic growth, there has been a drive from UK Governments for decades to deindustrialise, and we now have some of the highest industrial energy prices in the world. It's scarcely surprising if we aren't seeing economic growth, given that the main growth areas are in non-jobs such as EDI and zero-carbon which produce nothing tangible. And all these alleged 'green jobs' which they talk about haven't materialised - what a surprise.
The problems with the country aren't that we are under taxed - it's down to decades of inept government, with no signs of this changing.