What cut of beef is a "Beef Roasting Joint"?

Topside, silverside. Too lean to make a good 'pulled' anything. Supermarkets are poor for long-cook beef generally - for some reason they always want to sell you 'lean stewing steak' which will overcook like pulled shoelaces as soon as look at it. I often fall for it, then occasionally come to my senses & do it properly. Sometimes they'll do actual braising steak, but it will be too thin cut to be useful. This is one thing you really should get to a proper butchers for. Tell them what you're making & they'll know exactly what to offer you. Chuck & blade (from the shoulder), braising steak tends to be low in visible fat but high in collagen, perfect for this. Get them to cut you a thick chunk if you want to pull it, so you get long fibres. Butchers' stewing steak is cheaper but fattier & won't be as good for this.