I think the King/Queen was a figurehead even in 1776. The last time they were true autocrats was King Richard Coeur de Lion. Legally, King John became less all-powerful post the Magna carta. But the Hanoverians weakened the power of the throne (which had already been weakened thanks to Cromwell cutting off heads) by speaking mostly German
While the Magna Carta is one of the most significant legal documents in Western civilization and forms the early basis of constitutional governance, King John while agreeing to it under duress and to avoid being disposed, it was only in effect for about three months until John ignored and annulled it. He reneged on the most crucial section of the document, now known as Clause 61, as soon as the barons left London. It was also not the first attempt at curbing the monarch’s authority. The Charter of Liberties was a written proclamation issued by John’s ancestor Henry I when he took the throne in 1100, which sought to bind the King to certain laws regarding the treatment of church officials and nobles and was in many ways a precursor to the Magna Carta.
After John’s death, his son and heir, Henry III, clinched a victory over the barons at the Battles of Lincoln and Dover in 1217. However, to avoid a repeat of the rebellion, the failed Magna Carta agreement was reinstated by William Marshal, the young Henry’s protector, as the Charter of Liberties – a concession to the barons. This version of the charter was edited to include 42 rather than 61 clauses, with clause 61 being notably absent ("clause 61", or the "security clause", a council of 25 barons would be created to monitor and ensure John's future adherence to the charter. If John did not conform to the charter within 40 days of being notified of a transgression by the council, the 25 barons were empowered by clause 61 to seize John's castles and lands until, in their judgement, amends had been made.)
I think it is correct to say that many English monarchs following John could be fairly described as “true autocrats”. Henry VIII would certainly count as a despotic and tyrannical king as would Richard III even as some claim he was much maligned.
As for the Hanoverians weakening the power of the throne by speaking mostly German, keep in mind that from the time of the Norman Conquest until the reign of Henry IV who was the first to speak English as his first language, the Kings of England exclusively spoke French.