Hello all, I have a 400D but the 17-55 kit lense is somewhat lacking. As of November I'll be heading out to NZ for a year for flight training so I want to upgrade my glass before I go; as you can imagine there's going to be a LOT to shoot

This will include landscapes and 'people' shots (social/casual day to day stuff).
I have a budget of around £400, and the dilemma now is whether to keep the 17-55 kit and get a Canon 10-22mm, or upgrade my zoom to a Canon 17-55 IS USM and do without a wide angle. Advice and opinions welcome!
Some follow up questions;
If I get the WA, can the 10-22mm be used on a cropped sensor up to max focal length (effectively 35mm) as a day to day lens? Would this be advisable considering I'll be using it on social events and casual shooting etc as well as landscapes?
Given 17-55 is more flexible, would it be more sensible to invest in the 'all rounder' zoom lens?
Thank you for anyone who's taken time to read and respond