Cocktail piano is a wonderful catch-all term for a cocktail of styles, popular, classical, jazz, musicals, folk, anything where the melody is not far away. Cocktail piano can give your event an extra pizazz. Let me know if there's a particular song that's important for your occasion.
If you don't have a grand piano its quite reasonable to hire one. The specialists in London are Jacques Samuels Pianos http://www.jspianos.com , 0207 723 8818 Contact them with a quote for your location. If space and budget do not permit a grand piano a digital one can be provided for a small charge.
I have played at the long-established London clubs such as the In and Out Club, St James’ Square, and the Caledonian Club, Belgrave Square. Venues also include the American Ambassador’s Residence in Regent’s Park and major London hotels such as the Conrad International in Chelsea Harbour, the Mayfair Hotel and the Westminster Marriot.