I'm not quite sure if you're describing a lisp or describing the mispronunciation of the letter 'R'
a lisp being when the letter 'S', is pronounced as a 'Th' sound; as in "thience forumth" (instead of Science forums)
The word you typed in your post - incompwehensiboo - suggests someone who cannot pronounce their Rs
Which is indeed more common in the UK than in the USA. The TV celebrity Jonathan Ross would be a chief exponent of this. he pronounces his own name more like Woss or Ywoss.
i don't know anyone with a lisp, but I do know a few who mispronounce their 'R's
I don't see how having an unconscious desire to talk baby talk, or to subconsciously be infantile would explain why English people
misspronounce their Rs
It's more likely that the way English people are taught to pronounce words beginning with R, to be the reason why they're more commonly mispronounced in England than in America.
The R sound in English - as in 'Really' or 'Ramp'- has the tongue lying quite flat at the back with the front of the tongue close to the front or the pallet, making it very similar to the way 'Y' is pronounced - as in 'Yacht' or 'Yesterday'. Whereas with an American accent that back of the tongue is closer to the pallet, distinguishing it from the 'Y' sound, so that children who are learning English in America would be less likely to mispronounce their 'R's
I'd also like to refute the allegation that The English have such a subconscious desire to talk baby talk, or to prefer somehow a more childlike state of mind.
As an English man who has spent a fair amount of time in the states; I've always found that Americans - although far friendlier, polite and sociable than their English counterparts - tend to have a more naive childlike persona and world view than the Brits.
A view echoed by other Brits who have returned from the states. It's more evident in U.S cities than in the small towns though.