most power companys have slightly increased rates for the prepayment meters both gas and electric, but check around as some suppliers may charge a standing charge as well, i know that british gas dont, but if you do choose them i also know that you will have to wait until you have had your first bill before they will swap you over, a word to the wise though, read your meters when you first get your keys and write the readings in your diary or somewhere safe as you never know when you might need them.you will also need to call which ever company currently supplys the property where your moving to to register your details until you decide who you want to supply you as it will take a minimum of 28 days to transfer you to another supplier even though you have just moved in,
hope this helps, reply to me as i know a lot about british gas and the pre pay meter side of things