As previously posts have mentioned - declare everything. They already know all about everyone's police record, and if there's anything that will prevent you from being granted a licence, you shouldn't make it to the interview. If you haven't declared things then you may come across as dishonest in the interview. Doesn't matter if your reference knows about it or not, mine never even got contacted.
Have a look at doing the CPSA shotgun skill course. It'll show the FLO that you've had some training on gun safety. At my interview, one of the questions I was asked what would I do in the event of a misfire. The amount of people I still see opening their gun immediately after a misfire is unreal.
The interview is quite relaxed. They're just making sure there's no red flags. So if you've got 15 crates of Stella in the corner of the room and pictures of cannabis leaves everywhere (not implying you do), questions are going to be asked.
Joining the CPSA or BASC is always a good idea, as there's insurance included in the membership fee.
Hopefully you application goes nice and smoothly. Due to the incident in Plymouth in 2021, the the police in my area have suspended all new applications for the moment while new guidelines are implemented.