You can almost always get at least a 6-week free subscription via slickdeals.net

If you do not mind missing an issue from time to time you can order these every six weeks...

Has it been everyone's experience that redeeming miles for a mag you are already subscribed to(like say, The Economist) will extend the subscription?

I have renewed my existing Economist subscription three separate times using miles with no difficulty. The last two were even using someone else's frequent flyer miles, as you have to option, at least with Northwest, to use miles for a gift subscription. In each case, after a few weeks, the expiration date on the address label incremented by a year.

AGS

I have also done this a couple of times. From my experience, as long as the name and address match a current subscription, it will renew automatically.

I used some AA miles just last month for The Economist. It was a mailing that provided a link. I am sorry I didn't keep the letter. I just checked "Magazines for Miles" on AAdvantage but didn't see it. Maybe if you poke around the net you can find it.

https://americanairlines.mpmvp.com/magazine/choose.asp

American is offering The Economist; note that it's alphabetized incorrectly, under T for 'The' instead of E for 'Economist.' Easy to miss! I'm going to use my husband's orphan miles for it -- a renewal, so after reading this thread I'm hopeful that the renewal will go smoothly.

Navigating through the Website, the path is

AAdvantage/Partners & Mileage Programs/Additional Partners/Magazines for Miles.

Do you know if any of these will allow you to have it mailed to an address outside the U.S.? (In this case, the UK...)

There is the possibility of getting the articles and audio edition online -- they haven't generally seemed willing to shift the subscription from the US to say Asia or South America or presumably Europe too, but no harm in calling up and trying your luck.