i watch GCN all the time (they are based just 10 miles from Maddy central in Bath)

It is possible but easily detected, it's more likely to happen in the amateur ranks than the Pro peloton. Pro bikes get checked quite thoroughly at every event whereas at amateur events there are few if any checks most of the time.

There will always be some participants, who may well be at a good level, who for whatever reason think cheating is legitimate, you need look no further than one LA from Texicana!