AT is relatively central, sandwiced between the M1 and M6 and easily accessible for most of the English and Welsh population who can get to it in under 4 hours, including the large population of London. You can easily M1 it from the east and much of the north. Now that the Oldbury viaduct works have finally finished, you can also spread traffic loads between the M5/M6 and M42/ M6 toll routes (I usually took the latter during the works that seemed to last forever).
Unless coming from the midlands, London, East coast, the south east or eastern parts of the South coast, Kent is either very far away or your options are very limited. I'm not saying there aren't large populations of people there to choose from - but you're talking way over 4 hours even if the stars align traffic wise from major cities like Leeds, Bradford, Manchester, Liverpool, Newcastle. That's before we even consider the journey for the Scots - it's a whole country away from them. Similar story from the west as your only options are A303/M3 (A303 at grade interchanges and Stonehenge bottleneck), M5/A40 (the less said about that route the better) or M4/M25 where hitting Heathrow for much of the day can easily rack up another hour to the journey making Bristol, Cardiff, Newport, Exeter, Gloucester less accessible as well.
Wherever a theme park is located, someone will always have to travel far but I don't understand the logic of building one so "major" in what is effectively a corner of the country that is the wrong side of London for huge parts of the domestic population. If it was more regional like Thorpe or Chessington then it would make sense as there's plenty of punters within easy reach but I don't understand why you would want to alienate such large swaths of the country in the North, West, Scotland and Wales?
Or does this show how rediculous the whole project was? An imaginary park so spectacular that people would travel from every corner of the country and continental Europe for multi day trips?