That's crazy. They are selling a inferior product at a more expensive price than their two big rivals in the UK (Kindle & Kobo)?!
If you really don't want to stick with Amazon, you could always try a Kobo Glo. It too is cheaper than the Sony at £99.
However, IMHO the best thing to do to avoid being 'tied' to a particular format is just pick whichever e-reader you think is best for you, and learn how to remove DRM from your ebooks and simply use Calibre to convert to whichever format you need.
Amazon's store wins hands down for me personally, but I also own a Sony reader and as I use Calibre to manage my library, it's only a couple of extra clicks to convert them for reading on my Sony. Even if I was to get rid of my Kindle, I'd still probably use Amazon more than other stores, but I'm not restricted now and won't be in future immaterial of what e-reader I use.