German Gymnasium's new "Berlin Hamper" is delightful. You can get one to take away, but I'd recommend staying and eating it on their terrace. The Grade II-listed building is beautiful, and was, I believe, originally London's first gym (hence the name), and they really go all out, putting down a special faux-grass table topper for you. The picnic itself comes with three sandwiches to share, crisps, sweet Bavarian Giga rolls, marzipan treats, sunglasses, and bottle openers. Oh, and a full bottle of German wine (I recommend you go for the sparkling option).
Price: £48 for just a glass each, or from £64 for a bottle (I winced when I saw this price when I went for the picnic, but it's for two people, so only £32 each, which ain't bad for lunch plus a full bottle of wine)
When: Until 10 September
Where: King's Cross
Email email@example.com or call 0207 287 8000 to book