Even better, I think I have cracked it! I used Nook for PC for download the epub to my PC. On Windows 7 the epub gets saved to
C:\Users\user\Documents\My Barnes & Noble eBooks\your email address. As others have said, the epub is not encrypted so it is a simple matter to transfer to the Kobo.
Nook for PC is not (yet) supported in the UK so it is not listed as one of the supported devices when you purchase the subscription on the Nook UK web site. But you can download Nook for PC from the USA site (
http://www.barnesandnoble.com/u/nook-for-pc/379003591/) and it works fine. Also the subscription is not showing in My Library on the Nook web site, but I'm not worried about that. The main thing is that it appears in the Nook for PC library (under eMagazines).
This should work out better/cheaper than FW, where they stopped subscriptions some time ago, so I have had to buy each issue individually. Now I can have a cheaper subscription from B&N.
Also the epub format I think is very similar to the format FW was selling recently. The Asimov's epub on FW changed format internally a few months ago. The old format had much better use of css and was produced by converting from another format to epub using Calibre (obvious because of all of the Calibre styles in the css). The format changed a few months ago on FW, and the same new format is being used by B&N (presumably it is the supplier who changed the formatting). Now the formatting is more "primitive" - less css, no paragraph indents, limited use of style classes and no evidence of Calibre. I always do my own styling anyway, so I can use my existing styling that I have been using when I got the mag from FW.