Many perennial can still be sown now, if you have a cooler place for them to germinate (or wait till the heatwave recedes). This is from T&M, more knowledgeable than I in your northern airs:
Now is an ideal time to sow the seed of spring-flowering perennials in seed trays of good quality compost. Try Aquilegia, Bellis, Delphinium and Lupin .
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thank you, very helpful. I will definitely wait until after this hot spell.