Those beautiful dahlias grow in the Walled Garden in Fulham Palace, South West London. Fulham Palace used to be a residence of the bishops of London and some of them were keen on botany and plants. Bishop Compton was a successful botanist himself and was receiving seeds and cuttings from the overseas colonies to grow them back in London. It is said that first magnolia in Europe was growing in the Palace grounds.
There is a whole range of dahlias and I love them growing against the wall which makes a very nice background in the photos. The dark foliage is also very attractive making the flowers stand out.