Location
I assume the original stage play was set in England and perhaps it was easier to just dress up the backlot that way, but to be honest this would have worked equally well if located in (say) Connecticut with Bogart's journey's being to NYC rather than London. Finding a chemist (pharmacist) willing to provide stuff might have been more believable.
I missed the first twenty minutes of it on TCM today, was it ever mentioned WHY a clearly American couple lived in England?
At least they used a right hand drive car with genuine English registration!
Come on lads, bags of swank!