hello,
from my experience, there is usually a certain limit of costs a uni is ready to pay. In my situation it was 500 pounds. It said in the form that it should be the cheapest possible option (no matter the limit).
Try to find something as cheap as possible but they will understand if you pick something slightly more expensive if it's more convenient for you.
I don't know about the 5 days in advance travel. When I was traveling the best return flight option was 2 days after the interview and because I was traveling via a place where i studied before, they proposed I could stay one night with friends. (The uni I travelled to was also ready to pay some money for that night but I didn't dare to include it in the form).
Good luck