When can Chickens stay out

Hi,

I am new myself to keeping chickens but I have three chickens who have just turned eight weeks old last Sunday and they have been outside since I got them over four weeks ago. They appear to be coping fine outside in their coop. I have also recently started letting them have the run of the garden which they have been enjoying very much. They have bneen going back in their coop in the evening too with no issues though they havent been too keen coming out the last couple of days due to cold spell. I live in Welling