There is nothing to say a traveller has to have a return ticket to gain entry to the USA.
Obviously standing on US soil does not give to right if entry; first you will be "Welcomed" by Border Control - Immigration.
Anchorage is fairly laid back but is very busy with all sorts of travellers, Oil Industry Workers, etc. Primarily all travellers to the USA must state their intentions face to face, and will be expected to "Come in Peace" and to comply with their issued or internationally agreed Visa rights. There is a special arrangement with the UK and most EU states have standard tourist access, usually 3 months.
You will be called forward, (only cross the white line on the floor when you are requested) fingerprints and photograph will be taken, and you will be asked what is the purpose of your visit?
You will then explain in a bright and friendly manner even though you maybe very tired, and ready in a businesslike manner to evidence that you are simply a traveller, and be able to produce paperwork, emails, Hotel bookings, route, and tickets with date of disembarkation from Argentina.
Be able to answer, what happens if you break your leg? Do you know what you need to do to as an Emergency Overstayer?
Be professional when discussing your trip with them, logistics etc, not well I intend to get on a bike and head south!
If you have a career, business, studies, home, wife, children in your country of origin so much the better.
*Those intending to remain in the US conversely would try the double bluff and show up with a return ticket (often at little extra cost) but the authorities are up to that one, so, take each person on their merit.
If you hold a UK passport, are straight up, and can evidence a few simple questions, you should have no problem, to reiterate there is NO immigration law stating one must have a return ticket. However, if you are saying that you are awaiting funds to purchase one at a later date, or that you are not exactly sure where you will be leaving from you will have problems. You will be ushered to a room, will have to wait for your luggage, and when they are ready answer many more questions. Remember: Failing to prepare is Preparing to fail.
Enjoy your trip and keep us posted