You should not leave the UK until you have received your new Biometric Residence Permit which will be sent 1-2 weeks after the decision is made.
If you are not sure about your travel plans it may be better to make your application in your home country.
The Home Office advises that the process for postal applications normally takes up to 8 weeks but it can take longer so you may prefer to pay more for either the priority postal service or the in-person premium service.
The International Student Office can advise you on how to choose your method of application.
You should use that ATAS certificate in your visa application or at the border along with your short-term study letter, as long as the certificate reflects the research that you did (a new ATAS certificate must be obtained during your studies if your research changes).
It is not possible to complete your Ph D under the standard visitor (tourist) route owing to the restrictions of study with this type of permission.
If you are on another programme, it may be possible for the University to be able to support you in obtaining a new visa to complete your course - for instance, following a period of intermission - but it is unlikely that you would be able to apply for such a visa from inside the UK.
You should contact the International Student Office as soon as you become aware that you might need more time.
To request a new CAS number you should complete the Visa Extension Request Form and submit it, along with the required supplementary documentation outlined in the form, to [email protected]
If you are undertaking a course that is subject to ATAS clearance, you will require a new ATAS certificate before a CAS can be issued.