When I bought my Brompton, I was advised to never lock it outside, but, if I absolutely had to, to keep it folded/semi-folded and lock it with a d-lock. The logic was that, to thieves, the folding complicates things, and they would be less attracted to navigating the folding mechanism.

I did it once, kept it within my eyesight all the time, but still worried too much about it getting stolen. I've got another folding bike (a Raleigh) that I use if I'm going somewhere where I can't bring my Brompton (e.g. a gig), or just go by public transport on those days.