They are also advertising themselves on TV

https://www.gov.uk/government/collections/fit-note

A Doctor issues a fit note - whilst the quality of the consultation may be limited compared to an in person examination it is unfortunately still correct. If your employee works at a single location close to home you could question why they did not use the GP they are registered with.

Then again many GP surgeries have issues with cover meaning appointmnents are difficult to obtain - this is the market opportunity this service seeks to address

This is looking to be something which we are going to see more often, with people signing up online and getting Fit notes to take to work if they have been off work for longer than 7 Days. 

You can't really blame them when its near enough impossible in some places to get a doctotors appointment for at least a week or 2

You need to go to the Government website and read up all you can about these, Obviously keep an eye out for fraudulant issues and those that are continually having time off that it would be in the best interest to get a Doctors Report