Because a lot of money is transferred every day into the property market in the UK, and because it is seen as a relatively stable form of investment, it has long been a target for money launderers. This is why it is now required by law that checks like this AML identity check are done https://www.w2globaldata.com/regulatory-compliance-solutions-and-software/aml-id-checks/ (AML stands for anti-money laundering) to prevent this from going on.
When you buy a property, you will need to provide identification to your solicitor so that they are able to complete the check for you – you will need to provide proof of your identity with a passport, or a driving licence for example, as well as proof of your address, which can be done with documents like a utilities or council tax bill.
This is only part of the process however – in order to crack down on money laundering effectively, you will need to also provide proof of funding. So you will need to show evidence of how you obtained the funds to buy the property to ensure that it was done legally.
In most cases, this will be a bank statement showing incoming money, as well as a mortgage offer from a bank or a building society. If you are receiving the money as a gift, you will need to provide proof of this, and the same applies if you have been lucky enough to win some money to buy a property.