As I understand it, rather than issueing X number of MREs for Y number of days, the Brits issue Y number of these ration packs. It let's them save becuase they only put in one accessory packet (which is bigger than ours, I've been told), and some other things can be consolidate, shuffled and modified. Not having eaten or seen anything but the empty carton in person, I'm not entirely sure how much it varies.

mreinfo.com has some information.

Me, I like the Canadian IMPs when could get them.