Photobox have an option that allows you not to crop the image.
Basically you have to add the pic to your basket & then there is an option somewhere to edit.
From there you can change the size that you originally chose (so you can see which size/format works best) & then there's a tick box that basically will fit the picture to the size of paper you have chosen, so you end up with lots of white space that you can trim off (probably top & bottom)
I have used this before (and have not explained it very well!), but basically choose different sizes until you find one that works well with the vertical/horizontal image ratio that you have. Then trim the image when you get it.
SO, if you went for that offer Drac mentions you'd end up with a panorama 30" wide & X" high depending on your picture ratio with some white space to trim off.
More specialised printers will probably do the trimming etc. for you so you don't need to mess around with it.