I am pursuing a doctorate degree at a UK institution and need to extend my studies. My Tier 4 visa expires in December 2020. But my dependant's visa, who is with me in UK currently, ends in August 2020. As far as I know, I can only apply for extension up to 3 months before the visa expires. So, the earliest I can apply is September 30. But at that time, technically my dependant would have overstayed and the visa expired.
Does the 3-month rule apply only for new courses? I am continuing on with my current course, and my institution is happy to issue a new CAS. But I am just confused with this gap in visa expiration dates and how to manage it. Ideally, of course, I would prefer to avoid my partner leaving the UK and then coming back. Thanks in advance for any advice!