Aruncus will grow in full sun, no problem, but it does need a little moisture at the roots, it does far better with a little shade though, so I think it's the soil here that is the problem.

Lobelia Queen Victoria is best grown as a bog plant or pond marginal, so here again the soil is at the root od the problem.

Foxgloves depending on which ones you have again it could be the fact that they are in full sun, a lot of them are more at home with a little shade, and as with most tall plants they do need some moisture at the roots or they will droop dramatically.

You need to search out some plants that are really at home with hot dry conditions, but even they the plants will need watering until they are established.

I water a lot!

I have also planted 3 trees in an effort to provide some shade.

But I digress I may need (as you suggest) to seek out some alternative plants :(

The previous owners had an odd sort of sprinkler system (a hose with holes in!) I can see why now....

Watch out for the tree roots. They can take a lot of moisture out of the soil, making hot and dry even drier. How about Lavenders, Eryngium, perennial Salvias and Geraniums? Some bulbs like summer baking if they have enough moisture in spring.

Leave us to grow the moisture lovers! You can grow so many other plants that we in the cool north-west can't!

Maurice.