Woods around Ruislip Lido here: http://goo.gl/maps/j5qEp were my usual evening blast when I lived in Uxbridge and later Northwood.
The Strava heatmap shows roughly where various tracks go.
Else up the canal to Denham and beyond makes for a pretty pleasant bimble along in the evening.
Bucks Offroad Sportive goes from Denham (just the other side of the A40) – route map here or easy to find past riders’ GPX files http://www.bucksoffroadsportive.co.uk/downloads/Routes/2011/BOR_2011_All_Routes_Map_VA.pdf – bottom bit would make a good on/off route straight from Uxbridge, or drive NW to one of the villages at the other end and follow the route.
Swinley is fun but for evenings you’re at the mercy of the traffic – for a while I lived in Uxbridge and my office was at the other end of Nine Mile Ride (about 15 mins on from the Lookout) – could take a bit over an hour to get in, would usually be more like two at peak times. Lots of places where traffic gets bad.