After your exam RSGB forwards the results to OFCOM. OFCOM may do some validation, then will update the website to allow you to apply for a callsign, and send you a certificate.

You can logon to the OFCOM website and apply for your new callsign. To create an OFCOM account you'll need to know your candidate number which is printed on the certificate that OFCOM send out. For your Foundation exam your invigilator may know what your candidate number is allowing you to avoid waiting for the certificate from OFCOM.