Your provisional offer will inform you of the pre-employment checks. The security checks, which are undertaken by National Security Vetting (NSV) Agency, usually take the longest to complete so you will be invited to commence this aspect once you have been informed that you have completed your interview, as this reduces delays. An invitation to begin the security clearance process is not a guarantee that you will be given an offer.
All candidates will be required to undertake security clearances to the level the role requires prior to commencing employment. Assuming the candidate completes their security forms and references are returned in a timely fashion, and that no further checks or documents are required, we would expect the individual to be advised that we are in the position to make a formal offer of employment and start discussions to agree an appropriate starting date.