Would be nice to have a link for these leaked prices you speak of. But just because the RRP is £55 doesn't mean they'll be £55. It's a great way for the stores to say "save £15, look how generous we are!". £40 tops trust me, which is the maximum I'd be willing to pay.
Just because it's a popular franchise doesn't give it the right to be any more expensive... IF, however, you're asking how much one would pay for a hypothetical forward-in-time style finished copy out now, then probably about £200, play it, the sell it for more money before actual release and buy a regular £40 copy