Where are good birding locations in or near London?

Firstly in the London area itself. I would recommend both Hampstead Heath and Richmond Park. The nearest Underground stations to Hampstead Heath are Golders Green,Hampstead or Highgate. Its a huge area of grassland and woodland containing all the more common breeds of British birds plus the raptors Kestrels and Sparrowhawks and many other vistors at migration time in Spring and Autumn. If you are going to Richmond Park catch an SWT train from Waterloo Station or the District Line of the Underground; the park is about 1 mile from Richmond Station - walk through the shopping centre and up Richmond Hill. Richmond Park is a national nature reserve and is huge with many very old trees and as well the birds you’ll see on Hampstead Heath, it has a particularly large amount of Green Woodpeckers and is also the home of a huge colony of Green Parakeets which escaped from captivity over 20 years ago and have bred in Richmond Park with enormous success.
Outside London why not visit the Royal Society for the Protection of Birds reserve at Pulborough Brooks West Sussex. This is a wetland reserve at the foot of the chalk downland the South Downs. Its a beautiful location for birds favouring wetlands; you might also see the raptor the Buzzard,
and in May at least hear the Cuckoo. Take the train to Pulborough from London Victoria; they run every 30 minutes
(hourly on Sundays) and the journey takes 75 minutes; there is a public footpath from Pulborough Village to the reserve.
I would suggest obtaining Ordnance Survey Landranger Map
Sheet 197 ‘Chichester & the South Downs’

The Snowden Aviary is a sure thing if it viewing captured birds doesn’t violate the tenets of birdery!