...rolling stock servicing the SEN to L'pool line are dreadful and quite dated looking compared to the likes of LHR and STN trains. Also, it stops at too many stations. They need to address these issues or passengers with luggage will struggle.
LHR and STN are huge traffic providers and warrant specific airport services and rolling stock as part of a (supposedly) strategic transport policy. Southend Airport does not and never will. It is no more important (probably less so) in terms of design of service and rolling stock in the rail franchise that say Stratford with it's multi-modal interchanges and shopping centre. No rail company (or the Government) is ever going to make bespoke provision for the relatively small numbers using SEN compared to the vast majority of the customers who are commuters. You need to cram them in, keep your 'passengers in excess' score down and make best use of the limited infrastructure. Look at Stansted Express - that's been diluted over the years to a semi-fast service with generic design trains with extra luggage racks, as indeed is 50% of the Gatwick Express service now (through London/Brighton trains, stopping at LGW using, again, generic design Electrostar trains with extra luggage racks).