It IS illegal to sublet a council property , it is not yours to let it belongs to the council or housing association. The entire reason to have social housing would not exist if tenants could rent them out. It actually happened just across the road from me, a council tenant sub let his council house and moved to Ireland , when the council found out (which only took a couple of weeks) the people in it were evicted and the original tenant has a warrant for his arrest out on him.