It's not about a discount per se, Shannons don't do any official national 'club member' discounts. They used to but not anymore, I was told by a Shannons employee it all stopped because people were joining clubs to get a discount on insurance. It's about getting offered cover by them at all, you can be declined for not fitting their underwriting guidelines.
Shannons insure the 'motoring enthusiast'. It is their main underwriting criteria so you have to demonstrate that. Frequenting a forum of your chosen vehicle or even one you don't have (ie. your favourite or dream car) is one of the hundreds of ways you can show you are into your car(s). Going to car shows, watching car oriented TV shows, reading car magazines, watching motor racing are all ways to show you are into cars.