This depends on the specific project and whether it falls within the scope of the mandatory certification regime; for works requiring a Certificate of Compliance on Completion, occupying before it is validly registered can create legal and practical difficulties. This is a technical area, so you should get specific advice from your assigned certifier or a solicitor before moving in.
For buildings that fall within the scope of the Building Control (Amendment) Regulations 2014, the Certificate of Compliance on Completion is intended to be lodged and registered around the time construction is finished and before the building is occupied, since it represents the formal statutory confirmation that the works comply with the Building Regulations. Occupying a building before this certificate has been validly lodged and registered can create legal uncertainty and is generally discouraged, since it runs ahead of the process meant to confirm the building is safe and compliant to live in.
In practice, there can be a short administrative gap between practical completion and the certificate being processed and appearing on the statutory register, and the specifics of what is and is not acceptable during this window depend on the individual project, the terms of any mortgage or self-build loan drawdown conditions, and the advice of the assigned certifier and solicitor involved. Lenders in particular often require confirmation of a registered completion certificate, or specific undertakings, before releasing final drawdown funds, which can itself affect the practical timing of moving in.
Because the consequences of occupying prematurely, or of gaps in the certification record, can affect future sale, insurance, and mortgage matters, this is not a question to resolve informally; it should be discussed directly with your assigned certifier, solicitor, and lender before deciding on a move-in date.