I have the Juki F600 and am also in the UK. It's a lovely machine, but the price tag is a little scary!
I went from a cheapy Singer Talent from Argos, to the Juki, and there was a big difference between the two, but there is also a massive price difference! I had made a bargello quilt top on the Singer, but I couldn't use the machine to quilt it, because I couldn't get it under the presser foot (because of all the seams, it was very thick!) and the Juki handled it with no problems whatsoever!
I don't really know how much the box feed helps, but it very rarely tries to eat the edges of my fabric (the Singer used to try and eat every piece of fabric!) and I haven't noticed any uneven feeding of fabric. I'll see if I can find a picture of piecing done on the Juki for you!
I managed to get £100 off the price of my machine, so make sure you barter a bit if you are able to! If I lived closer, I'd bring my machine over for you to have a try on!