I believe Methanoid is referring to
this item in the For Sale forums - a prebuilt SFF based on 780G. As it's basically a combined Chassis and motherboard that he's asking £105 for it'd be a pretty big saving on doing it yourself (~ £47 based on your spec above - enough to fit the SSD into your budget
). At 275 x 357 x 90 it's slightly bigger than the compucase case you were looking at, but it will take a 5.25" Optical drive so that'll save you some more money.
You could simply stick in a processor (probably an Athlon II X2 for ~ £45) and some memory (which you appear to have already) plus an optical drive and hard drive and you're good to go. if the stock HSF on the Athlon X2s are anything like the one on my Sempron they'll be pretty low profile and fairly effective, saving you again as you won't need to buy an aftemarket HSF.
In fact, I've almost convinced myself to buy it (if I hadn't already rebuilt my HTPC, that is
). Meth: the invoice'll be in the post mate