As nobody else has answered I will suggest good old euphorbia amygdaloides robbiae. Because if that won't grow I can't think of anything else that will. In good soil it is a rampant spreader but in poor dry soil it only spreads slowly. I've grown it in deep shade under a sycamore in light sandy soil and at present have it under a mature conifer of some sort (possibly juniper) where the ground is powder dry, and once it got established it began to spread quite happily. But I don't know how it would be in the far North! They are cheap enough anyway so no harm in trying one ore two just to see. In fact I could send you a bit if you pm me.