Can Guided Access be used to limit access to one active web site in safari?

I don't know about Guided Access being used to do that, but you can certainly use Restrictions to do it:

Settings>General>Restrictions (turn it on and put in a password - don't forget the password!)

Then, in Restrictions, scroll down to Websites, click on Specific Websites Only, and put in the website that is allowed.

Cheers,

GB