Deep breaths!!!!
There is absolutely no need to do anything yet. I understand you wanting the cheapest possible supplier for you - but wait a couple of months after moving in. You won't lose too much money in this short period.
You will have enough to worry about - things that can't be postponed.
Ask the seller who supplies the gas, electricity and telephone. When you have the completion date and know when you will be moving in, contact these companies yourself and inform them of the PROPOSED moving in date. Agree to confirm within 7 days of this date whether you have moved in or not.
On the day you move in, take the meter readings.
It is far simpler than you imagine. Don't stress about it.
When you have settled in and have an accurate idea of your usage, then you can see about moving to different suppliers.