It isn't so much about the hours, but the length of the service which falls within the same set of regulations. If a bus service is more than 50 km / 31.07 miles long, drivers need to use a tachograph. When the rule was introduced, it affected a lot of existing bus services. The result was to either split services into individual non-connecting routes, or to interwork two or more services that effectively use the same route number. Internally, they would have different route numbers, but externally they would keep use the same number to make it easier for passengers.
It just comes down to how UK driving rules differ from the rest of the EU, in that we have two sets of regulations running side-by-side. It's just that domestic rules mean that a tacho isn't needed, because what the driver does is already printed on paper.