It seems the rules ref braces are not unique to joining the RAF
Jacko - check this American advice out and see if you could apply the same caveat for yourself when joining. Get your answers lined up from your dentist with time lines etc.
source
Answering Medical History Questions | Military.com
Braces (If you wear or are planning on obtaining braces for your teeth, have the orthodontist submit a letter stating that braces will be removed before active duty date; release form and sample format can be found in the Recruiter's Medical Guide.)