You showed us a picture of red ,black ,and unsleeved copper wires in a connector block. Did you make those connections into the connector blocks terminals ? If you did,you have done it wrong and connected the switched live to a neutral
You do realise that the captive nut can be moved upwards,away from the valve ,so the mating surfaces of the valve and tail come into contact with each other before the nut is lowered onto the thread of the valve ?
I can't see why you need tail extensions ,that would require you to put the mounting brackets ( I assume there are 4) in completely new ,higher, positions wouldn't it ?
If you assemble the brackets onto the rad and position the rad onto the valves, mark the wall with bracket positions,then...
The new switch needs to be earthed. The earth conductors that are currently terminated in the mounting box need to be connected to the earth terminal of the switchplate ,with a flying earth wire linking between the switchplate and mounting box earth terminals.
The brackets that hold the radiator can often be lowered by approximately 5 or 6 millimetres,as they have an elongated slot , through which the screw that holds the bracket to the wall fits. This allows several millimetres of adjustment.
Also ,is the pipework copper or plastic microbore ?