getting an agreement in principle

hello, partner and i are first time buyers and have a few questions!

1. I've read the halifax do a soft credit check for the aip, does this lower our credibility or is it a case of picking any bank?

2. I understand that when we have an aip we don't have to use their services and can go to another provider. Just checking i am correct on that?

3. What is the length of time we have the agreement before it expires?

Thank you :)