The thing you need to understand about car aircon is, that it is quite often the most overlooked part of the car when it comes to servicing.
Many dealerships don't service/regas car aircon themselves, they contract it out because in order to have the equipment and operate it, you need people who have been on specialist courses and obtain the correct certification and of course there's the whole thing of keeping/storing refrigerant etc. Aircon servicing through dealer networks as part of a normal service, isn't from my own experience, really offered. Some dealerships idea of a aircon service, is to stick one of those air conditioning cans behind the rear passenger seat for ten minutes. I know, I've seen it done.
Most people, quite rightly, will switch on the aircon, if it's blowing cold, fair enough, to them there isn't a problem. However, from personal experience, any car once it is over three to five years old, the aircon sadly becomes one of those things that is just forgotten about. Over time, refrigerant leaks out naturally through hose joints and it is not uncommon, depending on the size of vehicle, for cars that are advancing in years, to lose up to 30 percent of refrigerant year on year isn't uncommon. My own car is a good example, I have the aircon serviced annually, ever since I had my condenser/dryer replaced. I use the aircon 100 percent of the time, daily when I'm out and about and over the course of last year, it lost 33 percent of refrigerant. I had the system degassed/vacuumed and refilled (costs £30) and the system is absolutely freezing. On top of this, you don't know if your condenser has a problem. A leak test will show this up. Also, if a condenser does need replacement, always replace the receiver/dryer at the same time.
From a health perspective, it is also very worth while having the aircon system serviced correctly anyway. It will keep harmful bacteria to a minimum and of course it will be far more efficient if it is running correctly. I realise there are folk on here who state they never, ever have their aircon serviced, well clearly that is up to them, but I think even they would be surprised if they actually had their own system serviced correctly at just how much more efficient it would be.
As for taking the car to the likes of the very well known High Street names already mentioned, yes, well, go for it, but don't forget, very rarely are things offered totally for free. There's generally a catch in there somewhere which will involve parting with money at some point, especially if they make a good enough case and say that the system is empty, in which case they'll want your business. That's the way it works. Personally, I'd use a specialist place that does only aircon. In the link below, is an example of a specialist aircon only place.
http://www.autokool.co.uk/