They should have de-fueled it, and taken the hit on time and money.
The last time I was involved in de-fueling in BHX it took about 30mins to find an empty bowser. And there are plenty of fuel hungry aircraft in BHX in which you can empty a full bowser.
This sounds like a time and cost issue, probably mostly time. Crew hours, knock on delays, airport opening hours that sort of thing.
It was a cock up and Easy Jet should have put their hands up and got it sorted.