Depends on where you are. Richmond Park is good.  c. 7 mile circuit. 20mph speed limit. Occasional van but I've never met a lorry (yet) there. Have met deer though, both on the road and the off road track around the perimeter of the park.

As XCRunner says - Richmond Park is quite good but as it gets warmer and sunnier it gets extremely busy at weekends so it is best to get in quite early or in the evening. Womens racing at Crystal palace on tuesday evenings. Training and racing at Minute Park in Hillingdon as well. Or you could pop along to Herne Hill for a spot of track riding. Or ride out to the North Downs.