IMDb lists "Hellboy" as 3rd September. I know a lot of people who really wanted to see this, but are so annoyed by the delays to it's UK release that they are going to buy the R1 DVD as it costs little more than the price of 2 cinema tickets and will be available over a month earlier. (No doubt the distributors will be wondering why the UK release underperforms...)
"Butterfly Effect" is currently slated for 13th September.
I thought of that myself and apparently so have the film distributor as they have added a lot of extra protection to the region 1 DVD to make it play on as few UK dvd's as possible.
*checks a couple of sites* - all I can see is that it will be RCE, which works (apparently) by refusing to play if your player is set to R0 - if you manually or auto-switch, they should be fine. My player plays them, anyway :-). if you're worried, there's always the R3 release which is out a few days earlier and is cheaper, but may not be as well specced.