what potatos can you plant in july

Tarquin,
Select some of your potatoes you have just harvested, about the size of a small hens egg. Plant them in grow bags. If you get a frost in september you can take them inside.

I hesitate to question what Doctorbob1 says, but surely newly harvested early potatoes won't shoot. The ones you buy for late sowing are specially treated (seed potatoes kept in cold storage). After all, they don't start to produce shoots when they are stored until much later in the autumn or winter. The tuber in nature would only come back to life the next spring.
Perhaps someone else could advise whether they can be chilled in the fridge for a while and then planted out or some other treatment to achieve this?

Modern ways have come into practice. I have suffered with growing out in the past, leaving early potatoes in the ground. Tarquin would not have lifted mid season or late varieties. Duke of York is an old one I used in the past with good results. I would never stratify potatoes. I have kept new potatoes buried in a tin for up to 6 weeks and they still maintain a thin skin.