If you complete your course early because you finish before the end date on your CAS, either because you are finishing early or you are finishing your course at a lower level e.g. MEng to BEng, the University is required to notify the UKVI of this.  The UKVI will then shorten your visa because you cannot continue to hold a Student visa if you have finished studies.  

Early completion

When the notification is made to the UKVI regarding your completion of studies, the UKVI should curtail (cancel) your visa 4 months later.  Once notification is made we would advise against travelling if you are hoping to return to the UK.  This is because once the UKVI take curtailment action you cannot travel back into the UK if you are outside the UK, even if this is within the 4 months remaining on your visa.  The UKVI may not take immediate curtailment action following our notification, but it is risky to travel once our notification has been made if you are hoping to return to the UK. You will receive an email from the University confirming when notification has been made so we advise against travelling out of the UK after you have received this.