If you look closely at rock walls or hard substrates while diving on the NE coast of England, you'll probably see these small anemones... they are usually only a centimeter or two wide, and without a torch are easy to miss amidst the dead-mans fingers, urchins, and encrusting sponges. They can be very colorful, but that is a detail easily overlooked in the murk at 20 meters down.
Most of the anemone is actually stuffed into a crack in the rocks giving them a much flatter profile than the Plumose anemones (Metridium senile) that can be found nearby. The cracks can be quite narrow... compare for example the size of the 'head' and the width of the cracks in the image below.
So what are they? Well... thats a bit hard to say. They are almost certainly in the family Sagartiidae, and probably Sagartia ornata, but could also be small Sagartia elegans. The small size makes me think the former, but this is not an easy group to identify from photographs alone. Short of returning with a crowbar or a rock hammer to dig them out, the best I can do is appreciate the beautiful effect they have on the underwater surfaces.