oh dear harpie tricky MIL situation!
I think I'd appear to play along/agree but to a limited extent... Something along the lines of "Oh yes don't worry, as soon as I have any kind of unusual or high risk scenario I will insist on getting the top consultant"
i.e. you are agreeing in principle about the consultant but only if there is a high risk situation - which hopefully won't happen and even if it does you don't have to tell MIL. Would that satisfy her do you think?
Also, there's definitely a case that NHS is better in scary scenarios than private - since they have all the expensive kit (which private hosps don't) and since they see so many more patients and have the top experts.
Would these kind of responses work on her??