I don't really know what to expect from a London hostel right off the bat. My initial feeling is that if you run a good hostel with a good atmosphere there should be plenty of demand to fill your beds. Of course that's how every busy city looks from the outside, but the real situation may be quite different. There is a lot of competition in London, and many of the hostels are very large. If the average occupancy is 78%, that represents a whole lot of heads in beds. Your entire hostel would only be about 11% of an 800-bed competitor.
Here is an older thread with some interesting info on the initial occupancy rates from Wombats.