Summer flowering bulbs, provided you can plant them without damaging the roots of the poppy.

I cut mine back hard after flowering then they often send up a later second flush of flowers. the new growth is pretty anyway and that fills some of the space. Sow annuals around them too and that helps if you cant find the bulbs you want.

How about gladiolas?

Or scatter quick-growing Nigella (love-in-a mist), Dimorphotheca, Candytuft or Godetia . . . none of which grow to more than 12": did you want something taller?

Thanks for all your good advice.x