It is a fundamental aspect of the UK tax system that HMRC have enquiry powers and that those powers entitle HMRC to make reasonable requests for information.
However, the rules differ if a matter is not subject to a statutory enquiry. Outside a statutory enquiry, HMRC have more limited powers.
In the course of an enquiry by an HMRC officer, you are likely to be asked to provide information and documents. Almost always, the officer’s initial request will be on an informal basis. Very often, it will be tactically sensible to agree to the provision of the information without obliging the officer to issue a formal notice. That could have a significant bearing on any eventual penalty if tax is found to have been underpaid.