If you're really concerned about your weight, you should really see your GP who will advise and refer you to a dietician if necessary.
Fortisip and similar are usually given to the elderly who have inadequate nutritional intake. Something like a Complan shake is probably what you're looking for rather than a protein shake. It is a balanced shake intended to give the same nutrition as a light meal. In addition to your regular diet, it should add some extra calories. You're highly unlikely to be eligible for this on prescription although it is available to buy in most supermarkets and chemists.
If you are looking to build muscle tone, you might be better consulting the fitness thread, and trying some commercial protein shakes.