1. It could and should fail the MOT Test
2. If stopped (e.g. by VOSA spot check) you will probably not be allowed to tow any further, even to get your trailer/towed car home!
3. In the event of an accident, your insurance company would be within its rights to refuse a payout!
So its worth sticking to the rules!!!
You should look for the sticker shown here (the tow bar manufacturers name will vary, naturally)