There is clearly a difference between what the Act says is illegal, and what the authorities choose to enforce/prosecute.
Anyone watching enthuiasts at more or less any UK airport will see many using scanners to listen to ATC, and the FR24/ADSB etc websites have large numbers of UK contributors of data in order to give the coverage that they do. There don't appear to be any laws prohibiting the sale of scanners or home ADSB receivers in the UK, so enforcement of the Act looks to me to be a near impossible task.