London broil is a method of preparation not a cut. Years ago flank was typically used. Marinating first then broiling - or grilling is the usual flow. Top round has been common for some time. Unless it is marked I've no idea what you have.
I'd suggest bringing the roast to room temp over a couple or three hours. Grill at low temps, indirect, to allow the meat's temp to rise slowly. This will allow tenderizing enzymes in the meat to do their thing. When the meat nears the point when it will soon be done do a finishing sear. Rest 10-15 minutes. Slice thinly against the grain.