If you're staying at generator or yha hostels longer than a few days and need actual laundry (not sink washing), there's a 24-hour place on caledonian road near king's cross. Wash £4, dry £2, about 90 minutes total.

Not glamorous but clean and safe even at weird hours. Tesco express next door for meal deals while you wait. Saved my life when the hostel machine died during a 2-week london stay.