I am fairly sure it is not Papaver aculeatum, which has a fruiting body about 15 mm long, with 4-8 rays at the top. Papaver dubium has a fruiting body 15-25 mm long, with 6-10 rays at the top.
I was going for P. rhoeas because of the black parts of the flower but that would have a top-shaped to globular capsule. From the 'narrowly egg-shaped capsule it fits P. dubium but this has flowers that 'rarely have a dark spot at the base' . (Weeds of the South East). It is not P. hybridum which has a bristly capsule.