Cecconi's is the only place that I know of where you can get Prosecco on tap. That tidbit alone is enough to make me consider this place a pretty awesome restaurant. There's something about drawing sparkling wine from a tap that ought to put me off, but instead it just really pleases me - and it helps that the prosecco is good quality as well.
Situated behind Piccadilly in Mayfair, there's no mistaking that Cecconi's is a fancy Italian restaurant, complete with white-coated waiters. The food is top notch - I had a lovely lobster spaghetti and spiced tuna tartare - and the service is quite attentive. Although when I visited they were fully booked, we were helpfully placed at the marble bar, which has an impressive view over the restaurant and the full menu available.
The clientele is a lot of Mayfair and St James types after work, segueing into a well-to-do crowd for dinner after that. Smart dressing and the odd under-dressed tourist are par for the course, and there appear to be a lot of regulars. Of special note in the cocktail crowd are the prevalence of really decadent looking drinks, from strawberry mojitos to raspberry belinis.
One of my choices for a high-end Italian restaurant, also an option for a posh, if busy, after work cocktail option.