Question
I live in Devon and have a south facing garden. The subsoil is chalky while the topsoil is a clay loam with a ph level of 6.5. What plants will grow well and which plants should I avoid?

Answer
Brian,

As long as your drainage is reasonable good, there's not a lot you couldn't grow.

Southern exposure would make me stay away from plants that prefer shade, like Hosta, Vinca, Skimmia, some Camellias for example.

Since your soil is fairly close to a neutral pH, the plants that may give you some trouble are the acid-loving plants, like Rhododendrons, Camellias, and Azaleas.  These use almost as much iron as nitrogen, and that iron is most freely available to them in the 4.5-5.5 range.  But even this problem is solved with adding a little  ferrous sulfate to the soil for them.

I'd be glad to give you more plant suggestions, but let me know about drainage, any plants that have been stuggling, and what plants you're interested in generally...flowers, shrubs, ornamental trees, groundcovers, etc.

And my best advise for any garden would be to get in the habit of adding compost to the soil.

For what it's worth,
Mark in Portland