PF:
are you categorically stating that, let us say, a Spanish operator will not be able to operate an Airbus or a Boeing PMI/SEN/PMI and/or BCN/SEN/BCN etc?
All depends on their willingness to accept performance limitations. There are of course some examples e.g. Spanair A320s operating out of Southampton. In addition to the runway length Southend has quite a few obstacles under the climb-outs from both runways which are likely to be a factor.
NS