Detailed information on the transfer (a.k.a upgrade) from MPhil degree to PhD degree process can be found via UCL's Research Students: Upgrade from MPhil to PhD webpage (also see Read the upgrade guidelines). You can also find further information via UCL's Academic Manual: see Chapter 5 - Upgrade from MPhil to PhD.
Useful links:
Departmental Transfer Panel Report (BSH, EPH, and DAHR only, download guidance)
Departmental Transfer Panel Report (PCPH only, download guidance)