White dead nettle

Pure white in the December gloom.  It doesn't sting but the leaves are so like stinging nettles that people usually steer clear of the White Deadnettle.  

The Wildlife Trusts website says:

"Lots of different species of long-tongued insects visit the flowers of White Dead-nettle, including the Red Mason Bee, White-tailed Bumblebee and Burnished Brass Moth. The caterpillars of the Garden Tiger and Angle Shades moths feed on the leaves, as do Green Tortoise Beetles."

Bees land on the lip of the flower and then probe the long tube to get at the nectar.  While they do this, the stamens in the top petal are bent down to dust the bee with pollen, which she then carries to the next flower.  
Symbiosis:  the bee gets a treat and the plant gets pollinated.  Win-win.