Not in anyway saying its the case for this poster but I've 'heard stories' of a bloke who after getting done infringing no longer even switches the thing on...I'm sure he isnt alone
Bloody stupid but I'll bet its the number one reason for people not switching their XPDR to alt or even not switching it on at all
I don't know the rules in UK but, in USA, it is illegal not to operate an installed ADS-B Out system or transponder. The penalties for intentionally turning it off may be more severe then the penalities for the infraction you hoped to hide.
The only legal option in USA is to have the system removed from the aircraft and have the removal recorded in the aircraft log by a qualified person. Not an option for me as ADS-B Out is required a my base airport.