Stinking Hellebore (Hellebore foetidus) flowers early. The ones in the garden have been flowering before Christmas in recent years. No doubt this is due to London's microclimate and warmer winters in general. Mine are from cultivated stock though it is also a UK native. 

 There are not many plants with green flowers. The flowers and stems of H. foetidus are of a particularly luminous green rising up from rather shaggy foliage of a darker hue.