The slip photo at Blu-ray.com (a more recent mockup than Amazon's, with the correct PG certificate) shows it'll be No. 52. Go to http://www.blu-ray.com/movies/Frozen-Blu-ray/91512/ and click on "Slip" below the cover.
Funny that the UK's "Classics" spine numbers have one again wandered off the beaten path compared to the studio's official U.S. list. By that list Frozen is No. 53; apparently the difference this time is the 2011 Winnie the Pooh movie was skipped in the spine numbers. (They first got out of whack when Dinosaur, produced by an experimental CGI unit within Disney Animation, was counted in the U.S. but *not* the UK; the UK got back in line by counting The Wild even though that film was *NOT* produced by Disney at all.)