The week before last I went to see the Kensington roof gardens in London, and I can certainly recommend a visit.
To get there, take the tube to High Street Kensington, and turn right as you come out of the station (bonus: you’ll pass a branch of Kew). As soon as you pass the Gap, turn right – the gardens are above the building that Gap is in. You’ll walk down that street (Derry Street), thinking you’re in the wrong place because there are no signs up whatsoever. But there is a doorway on the right – you can see a reception desk through it. Go in there, say you’ve come to see the roof garden, and they will let you sign in and take the lift up to the top floor (it’s free!). There are directions here.
The gardens themselves are not the fanciest you’ll ever see, but they are very nice (and will be even nicer – I was there at the very end of February, so there are plenty of things that will grow as Spring moves on). And it’s a real treat to be somewhere so quiet in the middle of a busy part of London – we were the only people up there at first. A woman arrived a little later than us and was also excited to be in on this secret part of London.
There are some mandarin ducks to see, and a couple of flamingoes. If you’re planning a visit, phone ahead to check they are open – if there is a function on, they can close the gardens. It would be a great place to go with a picnic – and the London branch of Wholefoods is just a few yards away, so you could stock up there before going to the gardens.