You could go for the Yamaha S540, it does PAL and NTSC progressive scan, seen the multi region version for less than £100 online, then you can pick and choose your surround sound receiver and speaker package.
The Sony DAVS550 does not output a progressive scan signal, the 42WH36 will do a 'virtual' progressive scan on the image, but it won't be as good as a properly fed progressive scan feed.
In fact, I am not a big fan of the DAV S550, it's ok as a bedroom system, but not as a full blown living room set up, Sub is inactive, and doesn't have the power or accuracy of other speaker package subs. But hey, that is just my opinion.
I would then go for the Marantz SR4400 which is being sold for less than £250 right now, and the Yamaha NSP100 speaker system, which is very good for it's size. You might find the Yamaha 440 amp for a good price somewhere (cheaper than the Marantz SR4400), if so, that is another good option.
If you have to go with the all in one package route, the only one I know of that is a decent system with Progressive scan capability is the Jamo reviewed in this months What Hi-Fi magazine.
But that system is £600 rrp, (sure you could find it cheaper on the net).
With the Bristol Sound And Vision show coming up (
http://www.bristolshow.co.uk) it might be an idea having a gander around there for a good system.
Let us know how you get on.