More ideas, for plants rather than seeds
July is a great month for planting leeks, but they need to have been sown in April, perhaps potted on as well. I always aim to have some spare leek plants to pop in gaps that appear as late as August. A late planting may not grow large until early spring next year, which is fine because you will be very happy to eat them then.
Dwarf French beans as a second crop are best sown by the middle of June, to plant by early July and crop for six weeks or more from mid August.
For something different, sow Romanesco cauliflower in June, to plant in July. The heads are exquisitely beautiful, maturing in about October.