Hi Stewart,  

A French person living in Britain would try to pronounce the words Lancashire and Yorkshire like a native ( but with a French accent ) . A French person never having heard of these would pronounce the "i" as an "ee" " sound.

In the word "impossible " the sound "im " is a nasal sound and pronounced the same as "un" , "in", "ain" .

Hopes this helps! 

Hi Stewart - as Cécile has explained, a French native with a good grasp of English would pronounce an English place name as close to the correct English pronunciation as possible. We use state-of-the-art synthethic voices which are trained to speak using very large databases of experienced French natives narrating texts, and since "-shire" is not a sequence of letters that appears in any French words the synthetic voice learns to pronounce English place names in the same manner as the experienced narrator.
It's quite likely though that in France you might hear other pronunciation attempts from people who had less familiarity with how we pronounce our place names.
Hope that helps!