[whiteandnerdy]

They are not strictly speaking illegal in the UK, and were pretty common in the 2000s and early 2010s.

The law requires employers to pay minimum wage to anyone who is a worker (i.e. is doing work for the company) - but "sitting in" doesn't have to be paid. Exceptions are also made for volunteering, and placements as part of a university course (these can be unpaid).

Enforcement isn't consistently applied so some smaller outfits get away with it. The big FAANGs and other major companies pay.