I have hardy fuchsias in that position - the poppies finish just as the fuchsias really get going. You do have to be strict about removing the poppies when they've finished though as they get untidy.

If you have annuals which can go in late, they work very well. I have used Cosmos and they go from small pot to flowering in 3 weeks (and nice foliage to cover the withering poppy foliage.)

Many thanks, great advice!

What about Eryngium 'silver ghost' The silver thistles will set the colours of the poppies off really well.