It is a recommendation only, so you won't be fined, but during court proceedings failing to follow this or other recommendations in the highway code can increase your liability.
If its not a law, then I'm not sure it will have much impact in realworld cases. Any collision between a bicycle and a car door remains an impact between a moving vehicle on the road and a stationary vehicle (Technically the word is "allision" when a moving object strikes a fixed object). Longstanding presumptions will remain heavily weighted against the moving vehicle. And proving which hand was used would seem difficult even with blanket camera footage. Even if only a guideline, burden will be on the bicycle to prove that the guideline had been broken.