I think diabetes presents a problem not only for police but also private citizens applying for firearms certificates.
I recently renewed my FAC and was advised of the following:
1) The police now contact every applicant's GP and advise the GP that the applicant either has firearms (if it is a renewal) or is applying for firearms.
2) The GP is supposed to advise the police if there are any medical conditions which make it possible for the applicant to misuse a firearm, specifically where such misuse can result in harm to the applicant or to others.
3) Whilst the firearms officer did not go through an exhaustive list of conditions, he did mention depression and uncontrolled diabetes.
4) Whilst he wouldn't outright admit it, I don't think they can force the GP to disclose medical information like that BUT it will likely shift attention from the police to the GP if someone does go off the rails and shoots someone and the GP held back on information that the police could have acted on.
5) If the GP advises the police that the applicant is depressed, for example, the police will confiscate the applicant's guns pending improvement in his condition. I was told that there have been cases where people have lost their guns for a year or so, been re-evaluated medically and then cleared to have them back. My main concern with all this is that patients will not report depression or similar conditions to GPs because they now know it can result in confiscation of the guns. I'm not overly convinced a person could get them back either, I suspect the cases I was quoted by my firearms officer were in the minority...
There are several changes to the way they are doing things (this was a Met firearms officer). Another change of interest is the database in which your FAC particulars are recorded: this used to be a separate database from whatever police database is used to run checks when performing a stop or enquiry. Now the two are merged.
The implication is that if you have an FAC that hasn't been renewed in time and you have not advised the police that you have sold the guns OR you still in fact own them, then if you went out through passport control at the airport you have an automatic red flag due to the fact that the expired FAC is flagged up in the database straight away. The FAC is a 5 year timer, and when it counts down to zero you are instantly flagged as a person who is in illegal possession of firearms.