By current standards, the London services would seem to have been rather infrequent. The uplift to regular frequency services came around 1976, under the auspices of National Express, and the service at that time was named “The Red Dragon”. By that time the role played by the RELH coaches on the London service had diminished.
Several destinations along the south coast were served, and there were other routes to different parts of the south east Wales valleys. These are summarised on the timetable for the service from Treherbert. Connections between the various services were made at Aust interchange, at the east end of the original Severn Bridge.
The timetable suggests that the service from Treherbert ran to Brighton, but, to the best of my knowledge, the journeys generally only ran as far as Portsmouth. The journeys to and from Brighton would appear to have been provided by the Cheltenham service, that operated at the same times between Portsmouth and Salisbury.