EI/EIR have a contract clause on Scotland routes and overcoming this could be a problem but LHR will be guaranteed from next summer and may potentially be the only route in 2019.
Also there would be much more tangible benefits if the A321CEOs were deployed on LHR/CDG etc and the A321NEOS went to the likes of FAO etc. This is due to the increased fuel needed on those routes making it more economical.
From a fuel prospective yes however they will make more money flying NEO to LHR/AMS/CDG/FCO with J almost full. They will probally not do any med routes because the config in Y is more less an A320 and they will make little extra on the J class seats.