Hi.
Your class 1 will lapse (become invalid for exercising class 1 privileges) on the day before you had your last examination. Your Class 1 will then remain valid for exercising Class 2 privileges for a further 4 years. In other words, you could fly on a PPL with an expired Class 1 medical, so long as it was within 5 years of the initial examination (class 2 certificates are valid for 5 years).
I believe you can renew your class 1 medial after it has expired without having to do the initial examination again. I don't know the time limitations but I would suggest that you probably can't let it expire, wait 10 years, and then go for a renewal. Best idea is to ring the CAA and ask them.
However, if I were you, I would go and get it renewed anyway. That way you can continue to be confident in your health and ability to commence flying training once you've graduated. Otherwise, you could be busy looking for a graduate position. Sure, it is an added expense at an inopportune time of the year, but frankly, you'd better get used to that if you want to fly aeroplanes
V2