Flex is flexible. Cable is only designed to be bent into position, and then fixed there. Cable only has a single copper wire for each conductor (L, N and E) unless it's something esoteric like a high power cooker circuit; flex has lots of very thin wires which can happily bend back and forth without breaking.
The exception is the wiring from the wall to a free-standing electric cooker, which nearly always has to be (not very flexible) cable, because flex isn't made big enough. The wiring regulations allow this because it is considered to be fixed wiring, although it's just hanging there and the cooker can be slid in and out for cleaning or decorating. This offends me in some indefinable way.