Hi Clem,
The US rat pack or "MRE" (meals rejected by everyone) are not the same as the Brit 24hr packs as they are just one meal per pack. You'd need (at least) 3 of them for a 24hr period. The Brit packs are a complete 24hr set of breakfast, main meal with a pudding and a snack type lunch meal of biscuits and pate. They also contain a brew kit (including the best hot chocolate in the world), a cold drink orange or lemon powder and some bog paper/tissues, matches and other odds & ends.
The MRE is always a main meal, usually accompanied by some dry crackers, with some jam or peanut butter and some coffee. There’s usually a tiny bottle of Tabasco sauce to zing things up a bit. There's no breakfast / lunch / evening meal as such, just pot luck as to what you get - spicy chicken & rice for breakfast - Yum Yum! They also come with an add water self heating pack, whereas the Brit packs need a stove to warm them through, although both Brit & US can be eaten cold if needs be. You get a free spoon with the MRE!
The MREs are a nice change, but I think the Brit ratpacks are better value.
As for dates, the Brit ones should have a "packed by" date on them and I think they're supposed to be used within 5 years. That said, I'm eating stuff that’s 12 years old and it's fine! Don't know about dates for the MREs, but someone out there should. I've eaten MREs that were 6 years old and they seemed ok with no ill effects.