If I remember rightly, Heston doesn't actually add salt to the whole burger. He basically cures a bit of the meat and adds that in with the fresh meat before mincing. The cured meat binds the fresh meat together without the need for egg, breadcrumbs or any other binding additive and provides seasoning for the burger, enhancing the flavour. Adding salt to the burger patty itself would be a bad thing if you plan to leave it in there, raw, for any length of time as the entire burger would cure, and as Haircut says, the texture would tighten.
Yianni Pappoutis (sp?) of The Meat Waggon advocates using coarsely minced chuck steak, aiming for about 80% meat, 20% fat. He uses thin patties cooked quickly on a very hot griddle, and dents the patty with his thumb when it first touches the grill to stop it from contracting in to a meatball.
Hawksmoor use off cuts from their steaks, bulked out with chuck steak. They remove as much natural fat as possible, replacing it with bone marrow (as it melts at a lower temperature than hard fat, allowing the burger to be done medium-rare without lumps of hard fat being present).
Heston, as above, minces fresh and cured meat together. He doesn't form patties, but rather lays the meat out on cling film as it comes through the mincer, rolls it in to a cylinder and then slices the cylinder in to individual patties. He says this gives his burgers a looser, more tender texture, while the cured meat helps keep the whole lot together.
Also, if you must cook your burgers medium-rare then be very careful. The worry about medium rare burgers comes from the presence of E. coli, Salmonella and Listeria. These bacteria live on the outside of cuts of beef and lamb, they cannot live in the inner tissues. As such, it is fine to have rare steak. The bacteria on the outside of the steak are killed during the cooking process. The same thing doesn't apply to burgers, once you mince the steak the bacteria could be everywhere. Burgers should be cooked through. There are, however, ways to do medium rare burgers with a minimal level of risk. The longer a surface of meat has been exposed to the air, the more risk there is of harmful bacteria being present. Trim all outer surfaces off of your meat, especially those facing outwards on the carcass (i.e. the surfaces of the meat that have been 'matured' for 28 days or whatever). Mince the meat at home using a freshly cleaned and sanitised mincer, and cook your burgers as soon as possible after mincing. Do all of that and medium rare shouldn't be too much of a risk.