i've had a few good recommendations from here, so I owe one back, its nothing exciting, profiled by all the guides, but seems to have been forgotten about. it deserves better, El Parador on Eversholt St, between Euston and Mornington Crescent. Great mix of old school and inventive tapas, well priced sherry and a garden. Lovely.
The Belvedere in Holland Park has a terrace upstairs with lovely views of the park and the food is pretty good.
So far as I remember a lot of the restaurants on Charlotte Street have tables outside.
Regents Park and St James's Park both have good restaurants.
In Putney the Phoenix on Lower Richmond Road has a pretty terrace and good modern, vaguely Italian food.
Lola Rojo in Northcote Road, Battersea, has outside seating at lunchtime. Delicious modern tapas, breakfast on Saturdays, snacky things like acorn-fed ham and other Spanish meats.
Franklins in Kennington has a nice courtyard and unpretentious food (the beef cheek, pig trotter and mushroom stew was delicious). .
Hush Brasserie has a pretty terrace - you can see a picture here:
If you want to watch tourists milling about, you can eat on the terrace at Chez Gerard in Covent Garden or, nearby, The Cove - which is the pub above a cornish pasty shop. It's not a restaurant, but you can order pasties from downstairs!
The Eagle on Farringdon Road is a gastro pub with a garden done out in mirrors, fairy lights and hand-painted glass. Again, it's not fine dining but they do decent burgers, calamari strips, Moroccan chicken sandwiches, etc.
Others that spring to mind are Le Pont de la Tour, Babylon @ the Roof Gardens, Le Coq d'Argent, and the Garden Cafe.
This link has some good ideas:
The Fox in Shoreditch has a nice outdoorsy courtyardy bit upstairs. It's pretty small though so make sure you specifically request it when booking!
I can also vouch for the Gun, a lovely terrace overlooking the O2 arena and the river.
My local Fox and Hounds in Battersea has a great garden too.
Olive Magazine recently had a feature on the best al fresco spots in London. I've not eaten at some of them _ and some I can't remember _ but off the top of my head:
1) Inn The Park, in St. James' Park. There's a covered terrace and two different dining experiences; one's more cafeteria-style while the other is more fine-dining.
2) Coq d'Argent, Poultry, above Bank station. Conran resto. I've had drinks and snacks here but never dinner. Weekend brunches are supposed to be a bit more reasonable.
3) Gun, Docklands. Never been but intend to try on a sunny day. Barbecue outside on weekends.
There was also a restaurant mentioned in Holland Park that I've not been to but which looked lovely. The Belvedere, maybe?