Eurolines run services from London to Calais in partnership with National Express, but the connecting journey from Glasgow would be between 16-18 hours each way. Also, French cigarettes are not as cheap as can be bought in Spain or Belgium.
If you want to go to Calais you should try contacting a local "day trip" firm, eg Parks of Hamilton. Otherwise, you might do better looking at budget flights from Prestwick to Charleroi (Brussels) or Girona (Barcelona) to take advantage of cheaper cigarette prices.