Trim excess exterior fat from the lamb, leaving a thin ⅛-inch (3 mm) layer. Butterfly the thickest sections of lamb by slicing horizontally parallel to the cutting board, unfolding meat flaps like a book to achieve a uniform 1-inch (25 mm) thickness. Whisk olive oil, lemon juice, garlic, honey, salt, pepper, oregano, smoked paprika, cumin, and chilli flakes in a large bowl. Rub marinade over lamb surfaces, cover, and refrigerate 30 minutes up to 8 hours. Remove lamb from refrigerator 30 minutes before grilling to take off chill.