My experience from the outside suggests that Easy and BA are running different yield curves.
Very simply, these days Easy seems to start lower and end higher than BA.
I'm sure it wasn't always this way - I used to do alot of winter sports LTN-GVA round trips a few years ago and never paid more than 100-150 on Easy. I saw
approx £400 on 'Orange' at Christmas.
Equally, perception that BA is 'premium service' is sometimes a bit misfounded...
You can land in the core LGW terminal on Easy, whilst your BA 'Premium' peers can find themselves in the expansion terminal at the end of the shuttle train, sometimes without even a proper gate (add a bus journey to the train journey to the terminal)
In terms of customer service, I've been flatly refused a refund for a BA flight they couldn't fly or offer a same day alternative to, whilst during the same ATC failure Easy were offering unequivocal refunds. The idea that BA will always look after you, and LCAs won't* is as much a myth that LCAs are always cheaper.
* Excluding Ryanair, who have a shocking attitude to all sorts of things, ranging from charging pilots for submitting CVs, near zero customer service staff, alleged repeated infringement of ATC airspeed limits, groundspeed limits...