I doubt there has ever been a potentially more exciting foreign collection of Art Exhibitions in London than there is now. I am planning a trip to them all and will let you know what I think.
Rodin at The Royal Academy
Holbein at Tate Britain
Leonado Da Vinci at The Victoria and Albert Museum
Velázquez at the National Gallery
(If you really want to get excited you could impress someone and visit the Turner Prize Exhibition.)