Yeah, the more usual wording is "given a free rein", which makes it a little clearer. A monarch isn't given their reign, it's theirs by right (well, with a bit of hand-waving. Or possibly sword-waving, but not since Henry VII).

The last English king to try to reign with no constraints was Charles I, it didn't end well for him.

James II entertained the same delusions as Charles of being absolute monarch. He had a kinder fate, being carefully allowed to escape twice on his journey to France.

For English kings you forgot William III

Actually I think the last king to go into battle at all was George II.

Giving 'free rein' comes from the pragmatic practice of allowing a horse to chose the best path over difficult terrain on the grounds that the animal would know better than the rider.