My neighbour swap their licence plate each night: is this legal?

Could be a lot more serious than that! Last summer my mum got a knock at the door, opened up to find 4 police cars parked outside, and officers urgently asking questions about my parents' car - where it was currently, who was driving it etc. etc. Turns out my dad, who had just pulled into the golf club for his weekly round, was being tailed by armed officers because "their car" had been involved in very serious crimes (which they couldn't divulge) but he didn't match the description of the driver/potentially armed suspect so they sent a bunch of officers to the house to verify who the owner of the car was before engaging with the driver. They had my mum ring him and give him a password to repeat to the officers tailing him, and she was told to instruct him to open the car window and put his hands on the wheel, not make any sudden movements until the police could approach and he give them the password. Once they spoke to him as well it was clear that he wasn't who they were looking for, but whoever was committing the crimes had the same model and colour of car, and had cloned their plates. They ended up having to stick some decals to their car and send the police photos so they would be able to differentiate between my mum and dad driving to Tesco or the driving range (they are retired and lead a relatively boring life tbh!) and whatever the other guys were doing. Since then they've received a ton of parking and speeding tickets, as well as Dartford crossing charges all with pics of the other car, driver always wearing a hat and face mask, or a balaclava a couple of times so not identifiable at all. They've always been resolved as they have a reference from the police that they can quote each time. Actually quite a scary experience, especially when all those police showed up and my dad was being followed!