do more favs win on us racing or uk?

yes

In the U.S. the ave. win % of favs is consistently around 31%.In U.K. it's not so different.In handicaps it's,as you would expect,much lower and in non-hcps much higher but I suspect the overall differences are negligable.I think the important thing to know is which types of races  produce higher or lower averages.For example,races restricted to fillies and mares are far more common there and my experience is that such races are well below the average.
                   Roggrain.

What's the average pricing of a fave in US compared to that in UK though? The difference in win % of faves might not look so disproportionate in light of that!

From research I did ( albeit 10 years ago) the ave price of favs in U.S.is 8/5.Haven't researched here but my gut reaction is that it's not that different.In any event I'm sure the big bookies will know what it is and will take it into account (of course,in their favour!)
Roggrain.