It depends on breed a little, and location / climate.
We have some Charollais in our breeding and find we need to be taking the entire lambs off this month - and we're in north Cumbria, so not far from you. When we just had Texels we could leave the entires on till mid-September - although having said that, we did have an unexpected and certainly unplanned New Year (Jan 4th) pair of lambs one year, which would have been an August tupping.
Separating male and female lambs from each other can often be left until a month or two later, if it helps, as the ewe lambs don't start cycling as early as the older ewes. But the male lambs will tup from 4 months old if they can find a receptive female!