I think this is a misconception arising from a "securitisation" structure that was used in the original debt.
"Securitisation" and "security" are two different things. I won't go into detail but in effect securitisation is a way of realising a capital sum up front from an income producing asset (ie season ticket sales). The security on the debt, however, will, unless its been drafted terribly, provide legal charges over just about every fixed asset of the group capable of being subject to a legal charge and will also include floating charges over everything else. That is what a debenture does and Aviva will most certainly have a debenture.
The question is who is party to the debenture, but again it would be extremely odd if all group companies were not part of the security structure.