Most through platforms at Reading do have intermediated stop boards.
Actually, all of them. The Western Sectional Appendix says:
READING
A ‘Rear Clear Marker’ board is a circular board with white background, a black triangle and numerals in black font.
These boards are located on platforms 7 to 15, they are positioned approximately halfway along the platform and are applicable to DMUs▸ and EMUs▸ with a consist of 2 to 5 vehicles. The boards apply to trains travelling in either Up and Down direction, and are reversing in the station.
Drivers, of DMU’s or EMU’s, with a consist of 2 to 5 vehicles, must stop the train 5 metres from the board if the service is to reverse in the platform and the next journey will be in the opposite direction to the arrival.
Drivers of formations that consist of more than 5 vehicles must go passed the ‘Rear Clear Marker’ board and stop the train at the normal stopping location for the formation they are driving.
Dated: 25/03/17
But that has nothing to do with whether permissive working is allowed - it only simplifies a train stopping at the first half of a platform. Permissive working is allowed in all platforms (1-15) other than
P12P11 in the Down direction, with (as far as I can see) the same rules.