Haha... I know your pain all too well... I've had to drop @Liam williams and @Katieb off in Gloucester on Theme Park trips before... Try day tripping Phantasialand from Plopsaland, then all the way back to Derby Via Gloucester in one day!!! And these thoosies say Ebbsfleet is hard to get to for a day trip... pffft, part timers... I bet the Americans reading this talk of 3-4 hour journeys being an issue are laughing their asses off at us.Fair enough... there could well have been congestion or an accident when I looked, but the distances to the London Resort were always longer than the other parks, for what it's worth.
Try living in Gloucestershire, less than 10 miles from the Welsh border, where pretty much no major UK park is less than a 2 hour drive away...
As for how the London Resort site stacks up for me; it's around 3 hours away on a good run (Maps suggests that it would be closer to 3.5 hours on a bad one), and I wager that if a park were to be built in that location, it would fall into the Blackpool Pleasure Beach-style category of "don't visit regularly because it's so far away" for us.
Thorpe is significantly closer, at only around 2 hours on a good run (it's one of the closest major parks to me), and even Chessington is only around 2h 15m on a good run.
I do concede that the train is a little easier... Ebbsfleet International is only around 2.5 hours from Bristol Parkway, which is actually less time than it takes to get to Chessington South (that's nearly 3 hours). Staines and Chertsey (Thorpe's nearest stations) are both closer, at only around 2 hours or slightly under from Bristol Parkway. That would involve a good 30-45 minute drive to Bristol Parkway in the first place, however, so it would ultimately take no less time than driving.
I think in summary, it's fair to say, yes, it would be a pain to get to by car, for a lot of the country, but so are other parks. However it's got good train connections, if we can sort out our rail networks operational issues in the next few years, travel by train wouldn't be a huge problem for much of the country. And finally, as a side note, lots of our easily accessible top class parks in the North / Midlands are very difficult for those in the South to get to, and that part of the country is one of the most densely populated areas of the UK, so probably deserves more top class parks than it currently has.