Considering there are ~49 million adults in the UK this surely has to be largest leak of personal data affecting British citizens.
Equifax stores the personal details of 44 million UK citizens. Information stolen includes names, addresses, DoB, & social security numbers. Other data stolen includes some drivers license details and credit card numbers. Many UK citizens will not realise their personal information has been stolen. Customers of many UK companies such as BT, British Gas & Capital One are thought to be amongst those who are affected.
Equifax discovered the breach of data which is thought to have occurred during the period mid May - July 2017 on July 29th, but have only this week disclosed details of the cyber attack to the public. The breach also affects ~143 million US customers.
More info - The Telegraph