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 certificates 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. It normally takes 3 business days for the portal to be updated, and Friday's are a particularly busy day for OFCOM (Due to a high rate of PMSE licences for weekends) which if you're impatient may factor into your decision as to which day to schedule your exam. Ensure that when you create your OFCOM account you use the same name you provided the RSGB, otherwise you will be unable to apply for your licence.