Redesign of end of garden in dry shade
I'm going to be a pain and complain about your replanting of a Leylandii. Yes it's going to hide the brutalised conifers behind, but you could have something much more ornamental such as a small group of birch or hazel, which would help create a mini-woodland garden and a cute setting for the tree house. With ferns planted underneath it would look nice although admittedly you'd have to eat into your lawn a bit.
I don't think Cape Plumbago would survive very well in a lot of UK gardens unless it was in a container so it could be moved around and then brought indoors during the winters. They are lovely plants though!