The National Audit Office has published a report on tax reliefs.
The UK tax system had 1,190 tax reliefs (as at October 2019). A tax relief reduces the tax an individual or business owes. There are two broad categories of tax reliefs: structural tax reliefs that are largely integral parts of the tax system and define the scope and structure of tax (such as the personal tax allowance); and non-structural tax reliefs where government opts not to collect tax to pursue social or economic objectives.
Non-structural tax reliefs are often referred to as ‘tax expenditures’. Examples include tax credits for companies’ research and development (R&D) costs and income tax relief on pension contributions. Some tax expenditures simply reflect a policy choice by ministers to support particular groups or sectors (for example the housing market), while others are designed to incentivise behaviour. Some tax reliefs can be difficult to classify because they have more than one objective and include elements of both tax expenditures and structural reliefs.
Tax expenditures are an important part of public policy design. They cover most areas of government activity, including welfare, housing, business, food, education, health and transport. They can also make the tax system more complicated and less transparent, and they could pose risks to public finances because their costs can rise beyond expectations. Tax expenditures differ from public spending in that they reduce the amount of tax collected, rather than consume resources after tax is collected. However, they are similar in that both affect the public purse and can be used to pursue discrete policy objectives. 
What is particularly interesting about this report is that it states that tax reliefs function in the same way as public spending, as of course they do. This raises the question as to whether all tax allowances and reliefs function in the same way as public spending: which of course they do. This suggests that the public expenditure figures should include the cost of all tax allowances and reliefs. If this were to be done then turning the Income Tax Personal Allowance into an unconditional income would not increase the stated public expenditure figure, which of course it shouldn’t, but now does.