I think you need to consider how heavy she is as well. The laws are very confusing cause it isn't just height and age that they consider but weight as well. The in-built harness aren't marketing hype and are about helping to keep our LO secure in thier seats as Hot1 said. The problem we had is that all the in-built harness we could find had a weight rating of 18 kgs and DS is heavier then that. I also recommend calling the police station as they are the ones who interpret the law.