One of the reasons it wasn't put in the UK was due to the lack of flat land. Which after last night's storm seems to be a good thing.
Back in the late eighties, early nineties, flying for a holiday was not a thing in Europe. Back then most people travelling for a holiday abroad took their car.
One of the reasons why Marne La Vallee was such a good location was it was within a four hour drive for 68 million people (and a 2 hour flight for 300 million people). London was not accessible by car from the mainland till 1994. My guess is, it would have done worse in the first years in visitor numbers, mainly relying on domestic tourism.
To give you a comparison, in 2019 3,5 million French people visited the UK, versus 13 million UK people visiting France. France in general is a more desirable vacation country than the UK, I think.
I am not sure how many Anti-Americanism feelings there would have been in the UK back then. For example, the French had a problem with the appearance code "an attack on individual liberty" (quite a funny thing to accuse the US from ;-) ), as children in the UK wear uniforms to school, this probably was less of a thing in the UK? The strict rules the TWDC had, made 10-25% of the staff resign in the first year. Would that have happened in the UK, maybe, maybe not. Yes, historically the ties between the UK and the US are stronger than between any other mainland European country. But those ties do not equal similar culture. I can imagine there would have been issues as well.
Not sure about 1985 when they were looking, but probably the term EURO-disney wouldn't have survived now ;-)
Being able to serve alcohol, mainly wine at dinner, was a thing for the French. Would it have been the same for the UK when it comes to beer? Sipping wine at dinner is different from downing a pint in a pub.
It is nice to speculate how the park would have been different.
We probably wouldn't have had Ratatouille, Hunchback of Notre Dame would not have been the theme of the 5th anniversary, There would probably be more Alice in Wonderland or Winnie the Pooh. More trains?
Most likely the park wouldn't have been bilingual.