By law, each Local Authority must have a 'Local Safeguarding Children Board' (LSCB). These boards are joint agency groups that are responsible for child welfare in their area. The LSCBs write policy guidelines for safeguarding children, ensure that the different agencies work together and conduct inquiries into serious cases or child deaths.
This is a comprehensive list of the LSCBs in England and Wales (Scotland and Northern Ireland has different arrangements). Note that although each London Borough has its own LSCB, there are pan-London protocols which can be accessed here: London LSCB (32 London Boroughs)
(Occasionally these links to each Local Safeguarding Children Board LSCB may change, if you find any that are broken, please let me know so that they can be corrected. Thanks.)