Two of the three subspecies are frequently found in Herefordshire, a slightly unusual situation, subspecies segetalis and subspecies nigra. They are sometimes tricky to name, but much more commonly partition well on the basis of their general appearance and habitat preferences.
Subspecies segetalis is a tall scrambler common throughout Britain. It reaches a metre but is commonly half a metre, supporting itself in tall vegetation with its tendrils. Its flowers are usually paired and have the standard paler than the keel. It is NEVER a component of traditionally managed, longstanding species-rich grassland, perhaps because continuity of cover by perennial plants has never allowed it, as an annual, to get a hold in the root zone and establish a meaningful seedbank. If found there it is a sure-fire indicator of recent disturbance.
Subspecies nigra is generally a small plant of short grassland flowering low down in the sward and not really needing much in the way of tendrils, getting support instead from its procumbent habit, resting against the open grasses and other herbs with which it grows. Its flowers may be paired in larger plants but more often seem to be borne singly with us, and are a brilliant uniform pink, like jewels in the grass. Unlike ssp segetalis, it is usually a component of at least half-decent grassland.
The third subspecies, sativa, also occurs in Herefordshire, but only as a rare casual. It is now included in some bee and winter birdseed cereal margin mixes sown within agri-environment schemes, so may increase. Note in the pic below that it has the same two-coloured flowers as segetalis. Only record this subspecies though if the plant strikes you as HUGE and if you are in the correct habitat, or if you are late enough in the year to see the ripe pods, usually hairy and pale yellowish brown, rather than hairless and black/dark.