You're presenting points as if they are fact when they are not. Shall we reduce it down to the very basics?
Caterham can keep up with traffic. FACT
Horse can't keep up with traffic. FACT.
Horse rider is aware his/her mount cannot keep up with traffic and is therefore an obstruction. FACT.
Horse rider therefore makes conscious decision to cause an obstruction. FACT.
Horse rider is therefore placing themselves above the other road users. FACT.
Drivers driving too slowly can and have been prosecuted for 'Inconsiderate driving'. FACT.
Therefore it is legally recognized that travelling on a road too slowly is inconsiderate. FACT.
I rest my case.
It is also legally recognised that horses can travel on our road. They should do what they can to cause as little inconvenience as possible, which everyone I have ever seen does (i.e. riding in single file, being visible)