At one point I too was under a mistaken impression about how long a validity period the paperwork had -- I think it's easy to become confused because there seem to be so many different "valid for" periods for different documents and even procedures, the vaccine, etc etc. It's a lot to keep track of; each step and each paper and each procedure has its own time clock ticking and all the validity times vary.
I think this aspect was, for me, THE most stressful because in all the confusion of the whole move anyway, it was very hard to keep track of all these different pieces of information and different deadlines for this or that. I had to write down absolutely everything, everything I'd researched, discovered, discussed in phone conversations with Defra or the USDA. It was mind boggling. The good news is, what seemed like a total mess to arrange -- and I did feel it was a mess I was failing to stay on top of! -- turned out smoothly in the end.