Not until all risk of frost has passed for both. You can start Dahlias off in pots or trays from Fbruary onwards as long as you have a frost-free g'house or other place to keep them.

Thanks ,very much appreciated

You can also take cuttings of the shoots that the Dahlias put out when they are 3 or 4 inches long. Then you'll have even more plants! Both will over-winter if covered with a deep mulch....although maybe not this winter!!

Getting dahlia tubers to shoot for the cuttings can be very difficult at this time of year and you need quite a bit of heat. I've had lots just rot, either because I've left them too wet, or too little heat, or both.
I've experimented over the years with sowing dahlias from seeds very early, and much later, and the same with starting tubers into growth. As dahlias are mostly late flowering plants, I don't think you gain anything by starting them too soon. Much better to wait until everything warms up in April. You will find both the cuttings and the seedlings catch up and even overtake the ones you started with so much trouble earlier.
I expect the 'professional' dahlia growers will disagree with me, but they are the ones who probably spend a lot on heat and light to get their plants off to a flying start.