The simple solution is to compose the image just the way you want it so
that no cropping is necessary before submitting the image for printing.
I routinely do that with standard images even with standard print sizes.

Within iPhoto (and probably most of the others), there's the option to
completely fill an entire page with one image.  You can use this
option to "cheat":

Step 1: In the template software, look at how the template software
creates its background, including white edged frames, shadows from
lifted corners, etc.  Your plan is to copy this stuff, so do some
screen captures, take a flatbed scan of a page from a previously
printed book, etc.

Step 2:  Go into Photoshop (or equivalent) and create a (large) blank
canvas.  You're going to be using this to design-your-own "big photo"
that consists of your desired compositional layout of one, two (or
several) of your non-standard panoramas or whatever.

Step 3:  Populate the blank canvas from Step 2.  Don't worry about the
background yet.  FYI, it is best to work in layers.  Also note that
you can also mix in some standard ratio stuff too, if you wish.    For
example, you could take two of your horizontal panoramics and copy/
paste them into your "big photo" with a gap separating them.

Step 4:  Using what you picked up in Step 1, go fill in the background
of your "big photo" from Step 3 with the same sort of background that
the template software uses.   Similarly, copy any white pictureframe
edges, etc.  Whatever you want.

At this point, you should have a saved image that looks like an entire
completed page out of the template software, with the exception being
whatever you chose to do layout-wise that is beyond what the template
software supports.

Step 5:   When you're satisfied with the results, save the image, then
flatten the image and use "Save As" to create a JPG (or whatever the
template software will support) for the next step.

Step 6:  Import your image from Step 5 into whatever template software
you're using.

Step 7:  Within the template software, go to the desired insertion
location, and choose the relevant "fill entire page with one picture"
template option.

Step 8:  Drag-n-Drop or otherwise insert the Step 5 completed image
into the  book location identified in Step 7.

Step 9.  Observe results & refine as necessary until you're done.

Overall, this sidesteps the template software's intended goal of
"trying to make things easier", but it is a viable work-around and is
reasonable to do when you only have a few special pages.  You'll want
to see the final results in a printed hardcopy before you know you'll
be fully satisfied with it.

-hh

I'm making a book right now with MPix.com--I don't know if they ship to
the UK or not.

   You can place any size or shape frame or photo on a page, and make it as
large or small or oblong or square as you like.

                               Chris