When can I plant Foxgloves outside?

They are biennial and will flower next year, the bumble bees will love you as they flower over a long period open from the lower bells upwards over several weeks. If you don't deadhead they make a mass of seedlings.ready for the next year.

I have just read that biennials should be planted out about now in their final positions, so that they have enough time for development before winter. I have grown lots of teasels from seed this year and planted them out just yesterday.

Hi David, I have also managed to grow some teasels from seed, where in your garden have you planted them, I think they grow very tall.

Plant them now so that they can develop a good crown ready for flowering. Yes - they will flower next year

Do not worry too much about the continued growth of your foxglove seedlings. Simply plant them where you would like to see there beauty and then just leave them!!
They will flower next year, then either let them set seed naturally, and/or collect some and scatter as you would like them to grow.
In a year or two's time you will have more foxgloves than you think!

Personal experience of my favourite flowering plant.