One should have in mind that there are three variants of the Bordo out there: light, standard and Granit. Granit does not fit into the place behind the seat tube and is heavy, light is an invention to thieves. So possibly standard is the way to go for most people. You can however mount the Granit on top of the top tube or put any of the locks in the side pocket of a front bag. Trouble is: neither of them is very safe and one should have this in mind. Even the Granit version is easy to forcefully open if people know how to do it. When you look at the list of stolen Bromptons in the francophone Brompton forum you'll see that quite a few of them were secured with a Bordo.

I have been using the standard version from time to time on various bikes for a couple of years now but with the Brompton I prefer to not leave it alone in public places. In the end it is a folding bike - it goes with me where ever I go. Works flawlessly (even better with the rack-version the OP has bought) and I do it in 98% of the cases. Depending if bike-theft is an issue in your area or not this might be the best strategy. Once more the small footprint of the folded Brompton shines.