This was done with local buses, not intercity buses.

You could get much, much further using coach routes.

Not really. They are buses with multiple stops rather than direct city-to-city coach connections but the X buses (eg X6, X7 from the route in the OP) are intercity routes.

The long distance coach to e.g. Leicester starts at the same place, but takes the motorway and doesn't stop until it gets to Leicester. Maybe it will then continue to another city, e.g. Manchester. (I've done this trip.)

The OP took intercity / intertown "local" buses, but they stop at many larger villages and towns along the route. This is much slower, as it usually involves leaving the large road and winding through narrow roads to the centre.

Example map for the Leicester-Loughborough-Derby bus, showing a fair number of stops, where a coach service might make 1 stop between Leicester and Derby: https://www.trentbarton.co.uk/services/skylinkderby/maps-and...

The distinction is buses that a Bus Pass would or would not be valid on.