I will be flying through Iceland on my way to and from London. On my way back I have a 7 hour layover. I am interested in visiting the Blue Lagoon. I've been told there is a 20 minute shuttle directly from the airport to the Blue Lagoon. With time to clear customs to leave the airport, and time to get through security to return to the airport, is it worth trying to get to the lagoon for a quick-ish soak??