I'm currently using an NR Solas on the rear. I have the extended seatpost, so my seat doesn't touch the top of the seat post anyway.
It is incredibly bright, and cost about £9.99 in a sale on Planet X. They are still available, but now cost a little more:
http://www.planetx.co.uk/s?q=solas
On the front, I have two. One is a cheap Chinese Cree type one I got on
ebay. It's performed faultlessly, and still had charge in it this weekend when I refitted it, despite being off the bike since about March.
This is mounted on the left of the handlebar clamp and doesn't intefere with the fold at all (mount it to the right and it will hit the forks) and the battery is sitting in a modified drinks bottle on a Monkii Clip Cage.
Got it from here:
http://www.lightinthebox.com/4400ma...lver-headlight-headlamp-kit-set_p1021139.html
NB - it came with an American plug, so I had to buy a transformer too. It was dispatched really quickly, but I think it comes from the far east somewhere (despite appearing to be based in Holland) and took about a month to arrive...
As I was never sure whether that light would leave me stranded (their quality can be a bit iffy) I also bought a Lezyne 600 XL. I put this sideways at the top of the handlber stem and again this doesn't intefere with the fold. From memory this was about £40.
http://www.chainreactioncycles.com/...gclid=CI2Sxr3t984CFcEcGwoduq0Iww&gclsrc=aw.ds
Both front lights will light the road rather than just let you be seen and for ease of use I'd probably recommend the Lezyne as it's a little more user friendly and you don't have to worry about storing the battery pack anywhere.
My only gripe with it is that it doesn't grip the bars very well and ends up either pointing straight down, or too high and blinds people. It's better on the stem rather than the bars for staying put.