its an odd one, if you have no seat then there must be no belt, you cant just wip out the rears of passenger seats and leave the belt mech's they all must come out,
although same as everyone else ive never had an issue with an MOT station and FIA harnesses, most good MOT testers know how they should be mounted too, its bad mounting youll fail on rather than the harness
the interesting point as i was told buy willians the only reason there not legal is some muppet worded the EU Regs wrong, and they only put in push button and not buckle types