Heat your barbecue to high. Rub the steak with the oil, then season on both sides and set aside. To make the seaweed butter, blitz the butter with the seaweed and sea salt. Spoon onto some baking parchment and make it into a log shape by rolling it up and twisting the ends. Chill until needed.
When the coals are ready to cook on direct heat, hold the steak upright on the grill so the strip of fat down the side sizzles and crisps. Once it’s nicely browned, lay the steak flat on the grill so the striploin (the larger side) is closer to the flames, and the tenderloin is on the edge of the barbecue – this ensures you cook each side evenly. Cook for 5 mins, then flip the steak over and put it on the other side of the barbecue so the strip loin is again nearest to the flames. Cook for 5 mins more, then cover with a lid and cook for 4 mins further each side for medium rare. Put the steak on a plate, top with a large slice of the butter, cover with foil and leave to rest.
While the steak rests, cut some slices of seaweed butter and lay them on the flesh side of each lobster half, then put on the barbecue (tails shell-side down). Cook for 5-10 mins, depending on how hot the barbecue is and if the lobster is raw or cooked (see our ultimate guide to lobster if you're preparing it live). Wait until the shells turn orange-red – remove the tails when the meat is no longer translucent, but cook the claws for a few mins longer.
Serve the T-bone steak on a large board or serving platter, topped with or alongside the lobster tails and claws and more seaweed butter.