It is ridiculous; you can easiliy get smaller padstones, and 420 long is far too big for that load.
Its not the builder who's a dick - it's the SE.
Whether the pad stone is over sized or under sized by the SE is neither here nor there.
Providing you have fitted what is specified, then you have complied with your part of the contract. End of story
It is just as easy to fit a 440x215 as a 215x150.
OP Sliding a pad stone behind cable of light switch presents no problem.
Regards oldun