On the whole the deal4free prices follow the US very closely, but they will appear wildly out if the underlying size in the market is thin (this happens a lot with ICOS). If you look at a Level2 screen next to deal4free you can usually see why their prices are the way they are. That doesn't make it any easier when they requote you though, or just fill you at a stupid price without even bothering to requote!
However, a bigger issue using deal4free to trade US, is the very small range of stocks available through them.
H.