I prefer wired as it's potentially more reliable and accurate. I am very happy with the inexpensive Sigma BC1009 on my mountain bike but I can imagine problems setting it up on the Brompton with regards to folding and running the wire properly.
What have you other Brompton owners used successfully?
http://www.amazon.com/Sigma-BC-1009-...7069744&sr=1-1
I ride a BF, not a Brompton but that shouldn't effect which computer works well. Yes - wired is potentially a little more accurate because of occasional interference on wireless computers, but the wireless ones have gotten a lot better in the last few years and on any folder, the less wiring to get tangled when you fold, the better.
The main problem with wireless on folders is that the smaller wheel puts the transmitter further from the receiver, so you have to be sure the one you buy has a strong enough signal. In that regard and for accuracy, I've had a good experience with a Cateye Strada wireless.