For example, the r in "better" is not pronounced in British English. How about the "r" in "a better idea"?
Rhotic English is a term to describes varieties of English in which orthographic 'r' is usually pronounced, even at the end of a syllable. In non-rhotic varieties of English - such as Southern Standard British English - orthographic 'r' is only pronounced if followed by a vowel. It doesn't matter if there is a double /r/ or not in the orthography:
car / ka:
car park / ka: pa:k
car alarm /ka:r əla:m
carrot / kærət
Of course, whether this is actually a form of liaison or not depends on your theoretical phonological background. In order to be liaison, one would have to believe that there is some kind of underlying /r/ in the words where we don't pronounce it. This may or may not be true.
Hope this helps!