Welcome along and sorry it's not under happier circumstances - I guess it'd be inappropriate to ask for the customary introductory pics of your car.
Google "subaru hatch engine failure" - the results are not a good read. The dealers unfortunately seem to try anything to not honour warranties in many of the cases - they're already trying this with their claims about petrol used and oil levels. Unless the car has a data logger fitted, I don't know how they can prove fuel type etc.
However - I would think you might have a route via trading standards if you were not told it was an import when you bought it?
Does the V5 document state it's an import? (note - i'm not sure it would, but some import owners can maybe tell you)
Surely you've been driving around with void insurance because it's not declared as an import on your insurance? Many insurers won't touch an import.
Sounds awful fishy to me - and I think I'd be raging and checking out legal avenues if I was you because I suspect you're about to hit a brick wall with the dealership.
Note - Subaru/IM group are using the subaru forums to look for ways out of warranty claims - so watch you don't "incriminate" yourself by talking about the fuel you used or oil checks etc