The new Policing and Crime Act 2017 (PCA 2017) which came in to force in May 2017 changed that. Fully automatic Airsoft weapons now have a legal limit of 1.3 Joules and Single action/semi automatic airsoft weapons have a legal limit of 2.5 Joule. Above 2.5 J a single action / semi-auto legally becomes a an Air Weapon. Above 1.3J and capable of full auto an airsoft weapon becomes a Section 5 firearm. (Air weapons cannot be fully automatic without becoming a Section 5 firearm.) NI and Scotland have different rules.