Bills get split equally between all housemates. Trust me, it is the ONLY way to operate a shared house, or the arguments will be endless.
"But you take long showers!"
"But you run a PC all day!"
"But you're up all night and sleep all day so you use more lights!" ...
It gets ridiculous. Unless there are "special circumstances" - e.g. in my house, there were three large rooms and a box room, and we agreed that the person in the box room wouldn't pay gas/electricity (the room was genuinely tiny) - you just have to divide the bills equally.
Also, as your gas and electricity will probably be a monthly direct debit, you actually pay for more than you're using in summer and less than you're using in winter so that it spreads out over a year. So, even though this person isn't there now, you would actually in part be paying for her "winter usage".
You could just do it stealthily though. Don't pay those bills for a few months and build up a deficit, then set up the direct debit once everyone is in. The DD rate will be to cover usage and a little extra to cover the deficit you built up over summer. She'll never know.
Other bills like broadband split between that many people are really insignificant, you're talking a matter of a few pounds, so I'd be tempted to just let that go.