space your son sounds like the exception, but the majority of them are fronts for dirty money, same as car washes.
The ideal operation is cash only and deliberately sits just under the threshold for investigation. These are shops not rooms that are being used, so they're in £5k shopfitting before they kick off, then rental deposit, rent, utilities and non domestic rates. In @CBIT 's neck of the woods, that's probably £20k a year overheads. That's a shit ton of haircuts & wet shaves they need to do before they break even, and a fanciful amount to make serious profit.
There's one near me that has no discernible custom, and yet the two lads that run it have £30k wrist watches on and £250k worth of cars parked outside. Whenever I get stopped at the lights outside their shop, they're usually stood outside smoking or high fiving the local yoot.