The fact that nothing can be seen is actually the point of the video. It is perfectly possible for a driver to arrive, park up, and enter the store without seeing any of the signs of Horizon Parking, who manage the car park. If the vehicle then spends more than 3 hours in the car park, or any other of their Ts&Cs are broken then the driver gets sent a £70 charge. According to the British Parking Association Code of Practice, "You must use signs to make it easy for [drivers] to find out what your terms & conditions are".
The fact that nothing can be seen is actually the point of the video. It is perfectly possible for a driver to arrive, park up, and enter the store without seeing any of the signs of Horizon Parking, who manage the car park. If the vehicle then spends more than 3 hours in the car park, or any other of their Ts&Cs are broken then the driver gets sent a £70 charge. According to the British Parking Association Code of Practice, "You must use signs to make it easy for [drivers] to find out what your terms & conditions are".
0:42 - sign is on your left
The inside walls (especially those near the ways in) probably have signs but neither video shows these