Is it okay to travel during maternity leave? Will it breach any HR policies?

Maternity leave is for you to rest and take care of your baby after you give birth. Since you gave birth, then you are entitled to your maternity leave. It's like medical leave. If you have an illness, then you are entitled to medical leave. But being ill means you shouldn't be travelling, having given birth doesn't make you unfit for travel and you can't possibly lie about a maternity leave. If that makes sense.