When the last risk of frost is over - normally mid-May. You still might get caught out though - make sure you harden the plants off well before you put them outside.

Where you are, end of May, as for London.

Well, the Nursery in Ellesmere has not started selling the ready planted ones yet, so it is too early.
And out of interest, we are 17 miles up the road from you and the last date of frost here is July 7th! That was an exceptional year, but even so mid June frost is fairly normal for us.

We've had frost here in London on 4th June before, but it doesn't usually happen.