Fair enough. Presumably the operators all know this as well ... err, don't they ??
There also appears to be a bit of a mismatch in joined up thinking between TC and EGLL as well.
There's nothing in the Terminal Control MATS Pt 2 that says you must declare an emergency but it may be the case that the airport Pt 2 states that any hydraulic issue is to be classed, as far as they're concerned, as a Full Emergency.