Nasturtium is an excellent bee plant. I grow it around the edge of my veg beds to add a bit of colour & the bees love it. It's even started to show up in my honey analysis (it's part of the brassica family & look how they love OSR!).
For late summer, given your 'gap', you could try a border of late flowering summer perennials, that flower July - late Sept / early Oct. My bees love veronicastrum, thalictrum (common meadow rue is the best), echinacea, eryngium, echinops, sanguisorba, astilbe, agastache, purple loosetrife, tradescantia, helenium, rudbeckia & astrantia. Probably others, but these immediately spring to mind as the bees favourites.