Johnsons Photopia are the UK distributors of Wescott products - they're very helpful if you give them a bell I am sure they will advise.
The Wescott softboxes I've seen and played with have been pretty good quality. Also worth considering the Calumet Illuma range, which I have found to be better than the standard Bowens boxes in the studio and again very good value.
With Calumet boxes you buy the speedring fitting separately. As far as I know, this is also the case with the Wescotts, but give Johnsons a call to find out.
http://www.johnsons-photopia.co.uk/index.php?mode=view&id=1149
and there is a price list here:
http://www.johnsons-photopia.net/media/jpd_april13-pricelists/westcott-retail_pricelist.pdf
With regards to square, rectangular or octagonal boxes being "better" - it's all about what you're using them for and your personal taste. There's not really any significant difference between the square and octagonal ones in terms of quality of light, but some prefer the rounder catchlights in the subject's eyes rather than a square one. Just personal taste really on the whole.