There's no single, universally applicable "highest 11+ score." The 11+ exam varies significantly between different grammar schools and local authorities in England. Each exam has its own scoring system and the top score will depend on the specific test's structure, marking scheme, and the difficulty of the papers that year. Some might use a percentage, others a raw score, and still others a standardised score.
Therefore, there's no absolute highest number. A child might achieve the highest possible mark in one area's 11+ but it wouldn't necessarily be comparable to the highest mark in another area.