Just had confirmation on the mortgage question. The owner is the individual, not the bank, which is considered simply to have a charge on the asset. As such, logically, and now confirmed, that charge is irrelevant for these purposes.
They want the value of a mortgaged UK property in total, and consider the owner to be the individual named on the deeds. In terms of "value", I'm just getting that checked now ...
edit: the value is ... the original purchase cost. So, it seems to me that for the first year it's whatever you paid for it whenever you paid for it. The problem comes in subsequent years because a property bought a while ago might well have risen 20,000 since the value given at the point of declaration.