Your selection interview is only valid for 6 months. Some AFCO's have interviews before medicals (valid for 12 months), it depends which is statistically the area in which most failures occur & therefore the most economic route to take in order of priority, depending on the geographic demographic trend.
If it transpires you fail your medical (Medical: £100 average cost), then there is little point wasting time, effort & money on interviewing you, writing your report, requesting references, paying for the PJFT etc., & requesting security clearance (Security Clearance: £2,500 average cost). The cost of recruiting the average Royal Marine is around £7,500 (ish) including advertising - this is before the 10's of thousands of £'s spent training before you can be deployed & perform the task which you are employed to do.
The good bit is that when you do get interviewed, you'll be 100% fit & ready to pass PRMC, so it effectively means you will spend less time waiting to join.
....this is what i did any how (this was avery long time ago by the way.....
1) Get loads of vodka
2) get a mate you trust with your life
3) Get blind drunk
4) Get some pliers
5) Pull all the braces out
6) Drink some more and dab up the blood
7) Sit back and and admire your dentistry work
Notes of warning: Dont get into a fight for a few days after, youll end up looking like me.
Dont go back to the dentist, they will be well pissed off