You tell the tax office of any unusual deductibles, and they give your employers a 'tax code' which tells them how much allowance to give you (but critically not why, to better protect your privacy).

Even this isn't needed in most cases; things are either done employer side (paid from pre-tax income, like pensions) or directly by the government (basic rate tax is semi-automatically added to charitable donations, so only higher rate taxpayers need to declare it, and they all fill in a short return anyway).