Let's start with the most fun stuff, the drinking!
I took the EuroStar from Paris and waited for Dayne, who was arriving on a later train, also from Paris (story for another time), at what is billed as the "Longest Champagne Bar" in Europe. Searcy's is good for a glass of bubbles and a perfect place to meet someone at St. Pancras station, but the bar wraps around in a typical U-shape and doesn't really seem all that long.
The first real cocktail stop we made was at the Nightjar Bar near Shoreditch. This is the most amazing spot, I'm truly in love with what they do. We sat up at the bar, and as it was right when they opened and not too busy yet, we had a great time chatting with the bartenders. Each cocktail is made expertly, but what really sets them apart are the accouterments. My summer punch was topped with a small bushel of summer fruit, all of it cascading down the the side of the glass. Another drink, containing egg white, had the half egg shell attached to the glass by a small clothes pin and in it was a small amount of grenadine so that I could sweeten the cocktail as I saw fit. All this garnishing does take time but we felt it was entirely worth the wait.
(*Update 7/29/12 ZTH was awarded "Best New Cocktail Bar" last night at the Tales of the Cocktail Spirited Awards. I'm not kidding when I say go there!)
A cocktail or a glass of champagne at the Connaught Bar, in the hotel of the same name, is really a wonderful afternoon treat. The Mayfair neighborhood is posh, the hotel is lovely and the customer service is top notch. This sweet spot is on my list to always hit in London.
(*Update 7/29/12 The Connaught Bar won "Best Cocktail Bar" at last night's Tales of the Cocktail Spirited Awards. That is in the WORLD. Put it on your list.)
On Old Compton Street is the mainstay of LAB (London Academy of Bartenders) which feels a little like a fun-house of cocktails. Pages and pages of drink options, many a bit on the sweet and/or wacky side, but it's a good stop if you are in the area and the bartenders are very nice and happy to give recommendations.
(*edit 7/29/12 Callooh Calley won "World's Best Cocktail Menu" at last night's Tales of the Cocktail Spirited Awards. See? They rock!)
If you have time while in London be sure to hit up The Whiskey Exchange, not just for whiskey lovers, this booze Disneyland is housed in Vinopolis and has amazing collections of wines, liquors and champagnes. They also have interactive displays and tastings that you can sign up for. And you can shop, there are many delicious things to shop for.
London photo album here