Research shown in Druckman and Jackson (2010):
A meal eaten out produces around 83% more greenhouse gas emissions than a meal eaten at home, if travel to the restaurant or canteen is included. If the travel to the restaurant or canteen is not included, then we estimate that a meal eaten out produces around 71% more greenhouse gas emissions than a meal eaten at home. This includes non-alcoholic drink but not alcoholic drinks. It must be remembered in these comparisons that we have, in theory, included, all the embedded emissions both for eating in and out of the home. In particular, for meals eaten in restaurants, pubs and canteens, we include, in theory, all upstream emissions associated with the establishment and the service provided, and this explains why emissions due to eating out are higher than those associated with eating in.
See Druckman, A., and T. Jackson (2010) An Exploration into the Carbon Footprint of UK Households, RESOLVE Working Paper Series 02-10, University of Surrey, November 2010. http://resolve.sustainablelifestyles.ac.uk/sites/default/files/RESOLVE_WP_02-10.pdf
 Travel for food to be purchased at home is included in both cases here.