Or if
If you can't cope with an 800 series train and your mother can't cope with a Pendolino, then the best option is to fly from London City. 50 minutes in the air then tram from EDI.
Or, if air isn't an option, but the journey could take all day, the 0906 London - York, for a Voyager northwards, would completely avoid 800-series units.
Later departures would be possible on '225' sets by changing at Leeds, rather than York.
Or, if an 'Adelante' would suffice, then Grand Central, changing at York.
It's also possible to go from St Pancras (change Derby, Chesterfield or Sheffield)
The link below shows options
avoiding LNER (i.e. not including the '225' options listed above), Avanti, Lumo, TPE:
fastjp.com
If you want to do a '225', then check the diagrams (or ask here) and a bit of tweaking with a journey planner could generate a through itinerary.