I would disagree with Nick about being able to see all of Venice in a day - I lived there for an entire summer and still missed several things I would have liked to have seen! However, if you need to insert a travel day in there, I'd do it at Venice's expense. The suggestions below are loosely grouped by neighborhood - be sure to check that everything is open on the days you want to visit, and swap out things you don't care to see. I'm sure others will have more ideas for things to see, especially in London!

DAY 1 visit the House of Parliament and Westminster Abbey, see the changing of the guard at Buckingham Palace,

DAY 2 spend the morning walking around Trafalgar Square, Piccadilly Circus and Covent Garden, have lunch before stopping by a museum (The British Museum?) for the afternoon, visit the West End for a play in the evening.

DAY 3 visit Shakespeare's Globe before walking along the South Bank to the London Eye. Go shopping at Harrods or have tea in the afternoon.

DAY 4 visit Saint Paul's Cathedral, Tower Of London and Tower Bridge

DAY 5 line up bright and early for the Doge's Palace, have lunch before visiting the Gallerie dell'Accademia or Peggy Guggenheim Collection, visit Basilica di Santa Maria della Salute and Punta della Dogana around sunset

DAY 6 start early again at Basilica di San Marco, walk across the Rialto Bridge and visit the nearby market, have lunch in San Polo before visiting the Scuola Grande Di San Rocco. Take in an evening concert at Teatro La Fenice or elsewhere.

DAY 7 visit Ca d'Oro, take a gondola ride (or a ride on the #1 Vaporetto if you're going budget), spend the afternoon getting good and lost. Best thing to do in Venice! Return to the Piazza San Marco after dusk for a romantic stroll.

DAY 8 visit the islands of Murano Island, Burano, and Torcello Island

London is huge. Venice is teeny tiny.  If I were you, I would spend 6 days in London, 2 days in Venice.  You can literally see every nook and cranny of Venice in a single day!