There is no national law to this effect relating to cars.
The Licensing Laws, concerning the sale and supply of alcohol could apply to operators of buses , coaches and stretch limousines but there is no national law , that I can find ,saying that a passenger on a bus or coach cannot drink alcohol when the vehicle is moving. It is possible that local authorities have such a rule. Milton Keynes, in February this year, drew up rules for stretch limousines including one barring the consumption of alcohol unless the vehicle was parked.
The car driver could be at risk of prosecution for failing to keep proper control of the car if they permitted the boozing passengers to interfere with that and its possible that both driver and passengers could be held responsible for causing any accident, both at civil and criminal law.