By Speak english, Nominative

The noun after the verb is to be in nominative case and not in accusative. I think you mean this to be true in the case of a pronoun after a linking verb only. Taking your "rule" literally, we must have They invited we. because the pronoun is after the verb, the conditions under which...

And who says all British magazines speak English correctly? My way of thinking about this one would be that we use the nominative case ('I', 'he', etc.) when the subordinate clause refers to its subject (sorry, I don't know how to say this in grammatically correct terms), as in 'It is I who...