It's not an SRG issue it's an issue of secondary employment. You have to obtain permission from your principle employer, ie NATS because they essentially pay you and you are their asset. If you are an ATCO at EGLL for instance and you do Wycombe Air Park on the side as many ATCO's used to, you are opening up the possibility of having a serious incident at Wycombe and having you Aerodrome Rating pulled or suspended resulting in you no longer being able to work at EGLL whilst the incident is investigated. Ie NATS loses you as an asset because your ATC Licence, which NATS probably paid for, is suspended. That is the reason.
If you wanted to drive cabs or do porn movies on the side thats ok, because any problems you get into are unlikely to effect your ATC availability.
There was also a questionable issue of time restraint if taking part in 2 occupations whose hours are limited by law, eg commercial flying and ATC. Your SCRATCOH days off are to rest and recuperate etc, unlikely if you are flying commercially for reward, ie payment. Flying likewise has hours restrictions for fatigue and SRG are likely to question your rest time off flying if whilst on rest you are doing CLN westbound on a Mon morning.