You are the person with experience in this area, so I'm happy to bow to your superior knowledge. All I know is that a couple of the banks offer rates which are supposedly 'first time buyer exclusives' which allegedly have lower rates than their standard high deposit offerings.
At low deposit values FTB rates can be better than standard rates, in my (admittedly limited) experience. Or indeed some deposit values are only available to 'FTBs'.
If you are talking about being able to apply for a particular bank's first-time buyer rates, the precise definition will vary from institution to institution, so I can only suggest that you contact them individually to ask. I vaguely remember seeing 'not holding a mortgage for 2 years' being...