This P86 tax form is used to notify HMRC that you have arrived in the UK or returning to the UK from another country. It should be completed carefully to ensure that the entries reflect the tax basis on which you intend to stay in the UK. There are a few tax planning opportunities here.
The official page for this form is below:
Associated with this form is the DOM1 at the same link. This DOM1 form is needed only if it is necessary to agree with HMRC your Domicile position. Domicile is presently quite tricky to change so you need to take advice if this applies to you.