With easyjet you most definitely will be using ERES
Aer Lingus will be Astral
The only thing is when handling charters or ad hocs whichever stobart has paid for. Ink DCS, altea or even possibly the ancient codeco.
As someone who joined an lgw handling agent with no previous airline experience, ERES is probably the easiest thing you will pick up.
Good luck wishestofly.