From my own experience, I'm sure that BA will pay the 600 Euros/passenger according to EC 261/2004. In early 2016, I was on the LHR-ORD flight that was close to Iceland when they realized that there was a problem with the landing gear. We managed to come back to LHR and land safely because there was no wind (the day before winds were at 50 miles/hour so who knows what the outcome would have been). After we flew back to ORD, I received an email from BA telling me they would reimburse any expense I had getting to my final destination in the US. Since the place I was visiting in the US paid for the Hilton at ORD and a limo to my final destination, I did not need their help there so just answered with my request for EC 261/2004. They did pay it.
A year later, I read that the problem was that during maintenance of the plane they forgot to put a rig pin. I still think it's funny that a certain UK prince refused to reboard the flight