DistantRumble
You are correct that the Air Navigation Order does not specifical ban smoking. However, under the Order it is an offence to fail to obey the lawful commands of the aircraft commander. Most (if not all) UK airlines have a no smoking policy which is enforced by the aircraft commander. Anyone smoking on a UK airline is therefore likely to be committing an offence which is (if I recall correctly) punishable by a fine of up to £5,000 on summary conviction or by up to 2 years imprisonment on conviction on indictment. Smoking in the toilet is can also be caught under the offence of endangering the safety of an aircraft.