M0 comprised sterling notes and coin in circulation outside the Bank of England (including those held in banks' and building societies' tills), and banks’ operational deposits with the Bank of England. M0 data differ from M4 and other balance sheet statistics because they were compiled as a monthly average of weekly observations. They also included all notes and coin in circulation, whereas M4 only includes those held by the non-bank non-building society private sector.

Source: https://www.bankofengland.co.uk/statistics/details/further-details-about-m0-data