Destiny: Rise of Iron will launch tomorrow, September 20, and will probably coincide with Destiny's Weekly Reset. This means that the times that Rise of Iron will launch are as follows:
UK - 10:00am
US East Coast - 5:00am
US West Coast - 2:00am
Western Europe - 11:00am
Australia East Coast - 7:00pm
More launch times around the world can be found here but those in the UK should mark 10:00am tomorrow in their calendar.