Scottish couple wanting to marry in England

Wondered if anyone could shed some light on this for us as we are very confused. We live in Scotland but would like to get married in London. Have been told one or both of us would have to live in England for 8 days before making an appointment to register our intent. However, have also read there is a document we could get from Scotland to say we have permission to marry in England called a certificate of no impediment. Does anyone know what the right thing to do is?