Being too lazy to make myself a sandwich today, I went out for a so-called Reuben. There in the bagel shop what looked like a mother and daughter were having a disagreement. The daughter was pronouncing castle like I would--with the same 'a' sound as in bat. The mother took her to task for this--saying it is pronounced with an 'ah' sound (like in father).
The daughter's response was one that an American (at least not outside eastern New England) would never say: "It's not 'cah-stle' because it doesn't have an 'r' in it!"
(The mother went on to take the daughter to task for trying to be someone she isn't. Unfortunately, I missed the relevant bit when she said "You were born in Brighton, not [inaudible]!" Could have been someplace in the US, could have been someplace in the UK--since the 'ah' vowel-before-/s/ 'rule' is a particularly southern/'standard' English kind of thing.)
Americans who speak dialects with post-vocalic (i.e. 'after vowel') /r/s tend to think of ('standard') BrE accents as 'leaving out the /r/s'. But the daughter's response here gives an indication that that's not how BrE speakers perceive their dialects. Instead, they perceive the /r/ as being part of the vowel sound. (Really, the /r/ and the vowel preceding it are pretty much a single merged-together sound in any dialect--that's the nature of /r/. It's also one of the things that bad impersonations of AmE accents tend to get wrong--e.g. saying farm as fah-rrm. I had an old SAfE-speaking boss whose impersonation of AmE accents was spot-on except for that detail.)
And all this goes back to the previous post from today, about tot and that it's pronounced differently from tart. AmE speakers may think of a ('standard') BrE pronunciation of tart as 'not having an /r/'. But that's not necessarily how BrE speakers are perceiving (or even pronouncing) it.