The Moultons are VERY odd looking but a couple do fall within this price bracket, so might consider them. What are they like?
Not good for a busy train, but they ride great. I would not consider them for your intended use.
I'm edging towards a Brompton LX (mod'd to 6 gears), Mezzo D9 or Birdy (Red or City, not sure) or Dahon Jetstream P8 or XP - very nice looking in photos.
I don't think you can get a Jetstream XP in the UK, but the P8 looks good value for money. Do the roads warrant that degree of suspension though? It is also a larger folded package.
I would give the Brompton a la carte system quite a bit of thought (and ideally, test ride a few off-the-shelf configurations). I noticed no difference in the ride or handling characteristics of a monocoque Birdy and the older Red. The issue between them is whether you want a hub or derailleur system. The Mezzo's under gearing will only really matter on downhills - but you could freewheel once you got to 35mph or so.