While I have not done what you are looking to do, and others on the forum have done it many times, I have explored it in a slightly different manner.
What I found, with few exceptions, was that all the Defenders >25 years old that were with my price expectation (<$15K) were simply poor condition old trucks. Akin to what a 25 year old F250 would be here in the US. However some had very nice new paint, which looked great in photos.
What I did find was that, for well within my budget, I could get a restored Series III in much better shape; but it just doesn't come with the Defender cache.
The variable in my experience, to what you are considering, is that I was looking in Costa Rica and Belize, vs UK.
That said, I've known of 3 guys who have brought series Landys in from Costa Rica with no problems registering in US, but each state is different. I'll be bringing in two this year.
If you are buying from an importer, be sure terms of sale are FOB US, customs cleared.
Something I've been warned about is that US customs are well aware that, (being so high value in US), non >25 year old Defenders are being altered overseas and imported into the US. And, as result, all Defenders imported are highly scrutinized by US customs; pretty much considered contraband until proven otherwise.
If confirmed as contraband they are confiscated, destroyed and the person importing is charged with a felony. There are Defenders out there that are registered, and appear, as being >25 years old but are not. I would not want to have the US customs be the bringer of that very bad news to me.... I've also heard of this going on with JDM super cars too, i.e Skylines and Evos.
Thus, this is a very good reason to apply a lot of caveat emptor when looking to import a vintage Defender. Also, no doubt others here on the forum have imported successfully many times and that there are lots of legit vintage defender out there--I'm just offering a risk data point for your consideration.
The Toltec Coffee fleet....
96 FZJ80: 3XL, lifted, and shaved
94 FZJ 80: our Costa Rican coffee and surf mobile
70 Series IIA 88: After 18 months of wrenching, its alive and legal to drive!
70 Series IIA 88: in US on H-1B visa
56 Series I 86: a whole new type of rover hell....