If you want to use a drone in the UK for commercial purposes, it's a legal requirement that you have a 'Permission for Commercial Operations' (PfCO). This is granted by the Civil Aviation Authority (CAA).
In order to get your PfCO you'll have to demonstrate to the CAA that you can operate a drone safely in UK airspace (which is rather busy these days!) For more information on what this involves, and how you can apply, head on over to the 'Permissions and Exemptions' page on the CAA's website.
As part of your PfCO application, you need to demonstrate that you have compliant insurance for commercial flights. This is what a Flock Cover Note does, and you can get yours instantly emailed to you in just a few taps. There's no cost involved, and you don't need to make any long-term commitments!