The museum was until recently The Museum of Garden History, and it's located next to Lambeth Palace, on the south side of Lambeth Bridge, a stones throw from The House of Parliament. Housed in a massive church, it has just been revamped and looks amazing.
They have kicked of with an exhibition about the famous British gardener Beth Chatto, using one of my photographs of her lent by the National Portrait Gallery:
The graveyard surrounding the church has flowers growing in and around the stones, and is exceptionally pretty and photogenic. If you are ever in London it is well worth a visit, and it's really easy to get to, only 10 minutes walk from Parliament Square.