Once the building site has been completed, is it planned that Easyjet will own the land outright, or do they plan to sell the site to a property company (if Easyjet originally bought the site) and enter into a 10 year lease ?
Being the main tenant in an office building is one thing. Being the long term owner of the building is quite different. I imagine that Easyjet will be a very different company in 10 or 20 years time, and their needs for office space in 10+ years will also be rather different. Property companies are good at mamaging office space but lousy at flying; airlines are good at flying but not very good at managing office space when the size and shape of the airline changes significantly over time. Easyjet may well prefer a 10 year tenancy over building ownership