You could paint the arch and gate/side of the shed a seagrass colour (my favourite garden paint shade
) to brighten it up.
Add a climbing rose to the other side of the arch so you get a mix of honeysuckle and roses
.
I don't know how much sun it gets but there are some shade tolerant climbing roses available (even for north facing aspects).
I'd gently prune back the conifers so they're bushy on the other side but more clipped on your side. Unfortunately they won't grow back from the exposed twigs on your side but it should open up the area a little without compromising the windbreak hopefully.
If budget isn't an issue I'd get some tall sleeper raised beds in (at least 2 sleepers high) so the plants can benefit from better soil and you can perch on the sides of the beds 
I was just saying on another thread that Aldi have self watering troughs and planters instore and online at the moment in black/terracotta/dark green. They're plastic but a reasonable size and have a built in shelf to retain water and stop roots getting waterlogged. They're £3-4 each and home delivery is free over £20 on the website. Search 'self watering'. They're a godsend in hot weather and tolerate a bit of neglect!
Do you have a B&M with garden centre nearby? I went in one the other day and they had fatsias for £3.99 or 3 for £10 amongst other evergreen shrubs. Fatsias love shade, are evergreen and are great for impact while you fill in with smaller plants - they won't tolerate poor, dry soil though.
Lavender hates shade so I don't know how a pp is getting it to grow 
Buddleia thrives in poor soil. I think it prefers sun but seems to tolerate shade. It can run rampant though in the ground!
The water butt would be brill if you can manage it.