Some similarities to :
Limestone Woundwort (Stachys alpina) which is also softly hairy, the same 1m height and without rhizomes but it is not as hairy as
Downy Woundwort and the reddish-purple flowers are readily visible almost in their entirety. Limestone Woundwort is also as rare as Downy Woundwort, gaining an [RRR] moniker from Prof. Clive Stace.
Many similarities to : Lamb's-Ear (Stachys byzantica) which is even hairier than Downy Woundwort, but Downy Woundwort lacks underground rhizomes by which Lamb's-Ear spreads far for readily. The flowers are only slightly bigger (by up to 20%) than those of Downy Woundwort. Lamb's-Ear is not native, but rather a garden escape.
In the same Dead-nettle family of Lamiaceae (but in a differing genus) lies
Downy Hemp-nettle (Galeopsis segetum) but that only grows to half the height (50cm), the flowers are pale-yellow and a different shape, but the most important way to tell the difference is that this plant IS an extinct (in the UK) [E] and not just a very rare [RRR].
Downy Woundwort is a native plant lacking rhizomes and which grows in limestone soils but in so few areas as to be vulnerable to extinction. It grows in grassland, on the edge of woods and beside hedges