I got an MRI at 14 months. This was a standard part of the review process the NHS perform. I don't know that you'd get much joy pushing for one in advance. With the waiting list that may exist it could take some time to get your MRI and the timing may simply coincide with the normal one anyway. Still, it can't hurt to ask. Don't ask, don't get! BTW you may find that appointments are restricted because you'll need a specialized radiologist to review your case. I know I was put in with the top person at my hospital so it was a choice of one unearthly time on one day or an equivalent another day when they were in next.
The clunking in the knee could be due to imbalances in the muscles around the knee, especially if not accompanied by pain. Your physio should be able to run your through a few diagnostic tests to identify where the problem might be. Well, my first physio (lead physio at the EIS) did such tests. You then need to sort out a programme to regain muscle balance. Not all physios seem to do this, targeting major muscles rather than the smaller stabilizers. Changes should be seen within 6 months, but depending on the degree of loss it may take longer to get back to normal.
Sounds like you're good though. Have a good Christmas and don't get too drunk and fall down the stairs!