All online gambling operations in the jurisdiction of the United Kingdom have to request some form of ID from its user base. This is required by law and this rule is meant to prevent any underage persons from gambling online, as well as to prevent any potential money laundering schemes.
Identity verification is usually requested when the user wants to cash in their winnings. Usually, online casinos will ask for a government-issued ID card, bank statement, utility bill, or something along those lines.