Warm summer nights should be spent in the high grass in Hampstead Heath above the ponds. For midsummer bring herring, snaps and songs and stay until sunrise!
Appreciate the 'bar hidden away behind a bookcase' thing is pretty naff but the cocktails here are unreal (try the Hot and Cold), and it's inside Milroy's Soho, one of the most legendary spirits stores in the city
Rotorino is a lovely little Italian, intimate but casual spot in East London. The food is great and when the weather is good - having your wine and pasta outside is nice.