Again, I can't comment on conditions in the UK or other EU countries.
If the British government allows them to "transfer prices to reduce taxable UK profits, the government is at fault.
If working conditions are poor, are they any better UK wide in similar warehousing jobs?

Saw something on the BBC that this and similar tax practices were being examined by a Parliamentary committee of some sort. Has any change been enacted?

Not surprised that an "undercover reporter" found the work difficult- he would not be used to any kind of physical labor.

The 15% temp labor probably applies throughout the selling industries; stores here make most of their profit during the holiday season, when temporary hiring is increased.