You don't need to register your reader with Sony UK or get a Sony membership ID to use it. And unless you have already done so I would suggest that you do not install the software that came with it on the CD. the Sony eBook library UK version only takes you to Waterstones, where you can buy ebooks anyway without any registration (but you will need to download and install the Adobe digital editions software which you will find on Waterstones site or Adobe.com then register that with Adobe).
The classics on the Sony free disk are mostly available here on Mobileread and will be better formatted with less errors as they are lovingly hand crafted by MR members. (check the ebook link at the top of the page - you want LRF or Epub formats or maybe PDF (last choice) )
If you want access to the monthly free ebooks on the Sony store in the USA you need to download the US version of the
Sony eBook library software from
http://ebookstore.sony.com (click the download software link at the top of that page). This will give you the US version of Sony Ebook library, install that & register with the ebook store from within the eBook library - don't give any information as to your address nor under any circumstances give credit card details, (you don't need to to get the free books, but they are only technically available within the USA.) If you visit the store with your reader plugged in to the computer with the USB cord it will ask you if you want to register the reader, choose yes in this instance. This will allow you to download & read the $0.00 free books.
Ask questions of anything not clear or you don't understand one of our many friendly & helpfull members will respond.