The Royal Charter [http://www.ad.ic.ac.uk/secretariat/charter/index.htm#charter] and Statutes of the College [http://www.ad.ic.ac.uk/secretariat/charter/index.htm#icu] state that 'In so far as it shall further the educational purposes of the College, there shall be a Students' Union of the College (hereinafter referred to as "the Imperial College Union") for the benefit of the students of the College and in their interests as students.'
That's what makes us different from many other student unions - we are the same legal personality as the College.
Incidentally, the case in Leicester has bugger all to do with clubs and societies - it's all to do with Catering outlets being forced to charge VAT on drinks bought with food.
I never said the case in Leicester was to do with clubs and societies! If I remember correctly, the Leicester case was actually triggered by the Union trying to *recover* VAT it had already paid to Customs and Excise after deciding that actually it was exempt. The basis of the decision was that the union was entirely seperate from the university, and this has serious implications if applied across the country - although, as you correctly point out (and as I did in the article), ICU is defined as part of the college in the charter.
ICU is exempt - IC fulfils its requirements under the 1994 Education Act in a different way to the University of Leicester. It's one (single) advantage of our historical relationship with the Univerisity of London, as it now makes us safe in the future...