The biggest difference is the hourly labour rate. Main dealers carry showrooms, brand fees and larger overheads, and that goes on your bill before a spanner is picked up.
Independents also source parts more freely. We can fit manufacturer-grade parts without paying dealer-only prices, and we will tell you when a cheaper option is perfectly safe.
You can see who we are and how we work on our home page, and check exactly what we charge on the pricing page.
Thanks to block exemption rules, having your car serviced by an independent garage does not void your manufacturer's warranty, as long as the right parts and schedule are used.