I have known it snow in Somerset in June. My MIL (who had a pub) and I were doing a wedding reception in the village hall. It had been a lovely June day and I went in a summer dress and strappy sandals. As DH and I travelled from Wells to Radstock, the sky filled up and we got out of the car into a couple of inches of snow. We had a quick rethink and made a huge vat of mulled wine to warm up the wedding guests as they arrived.
Put mine in about a couple of weeks ago (Essex flatlands) before I went off to Cyprus for a weeks hols. Couldn't leave them in the greenhouse without watering for the week.
We had a local nursery who would never sell his bedding plants until 23rd May when according to him all frosts SHOULD be over. Mine have gone in this week.
Probably do mine this week (Sussex) Planted up some Petunia tubs last weekend and they're fine but the rest will go in this week and baskets planted up over the bank holiday weekend.
I planted some Lobelia in planters made from hollow tree stumps a couple of weeks ago, still looking good!.
Don't forget to water them in well, to give them a good start.
Now, or next week is a good time, but do be aware of the slug and snail invasion this year, worst that I can remember. Hate using pellets, but just 2 or 3 around each plant (especially Marigolds) is enough, with no danger to birds or pets.