Lots of venting might be a problem, most of the really light jackets I've been looking at strip away features to cut down the weight, so less zips and openings.
If you don't mind a smock look at the Rab Demand, nice design, good fit as long as you're a reasonably slim build, about 280g, and eVent fabric. They're £135 in Go Outdoors, and if you went down today there's an extra 15% off everything.
If you want a full zip then there's the Rab Demand, similar features, also eVent.
Haglofs do the Ozo smock in Goretex Paclite, super light at 180g, more expensive at about £180.
Rab, Montane, Haglofs – all do nice kit, you need to see what fits you, and is the best compromise of weight, features, durability, breathability for you.
For good info on lightweight walking gear have a look at this site, this guy is into walking, biking, camping and does good real world tests of kit
PTC