I interpret the AAIB report to say that both engines were functioning through most of the event. It was only during the approach to land that the right engine was shut down in response to the fire alert. The pilots were told by cabin crew that both engines had panels missing, but presumably had no reason to think that this was likely to dangerously compromise the engines.
So, for most of the flight the pilots had a reasonably stable two-engine plane whose main problem was going to be stopping after touchdown - is that right? Landing at Heathrow seems quite sensible to me in those circumstances.