Even in the worst-case scenario that she first started showing symptoms on the plane, there is minimal danger. The fact is that outside the three countries of Liberia, Guinea and Sierra Leone, there have been 35 cases only of Ebola in the world, and that number is mostly due to two small and soon-contained outbreaks in Nigeria and Mali. Empirically there has been no global spread of Ebola and little reason to expect this changing.
That Ebola has, finally, reached these shores is not really a surprise, but it's not really a threat either. If the total number of cases in the UK reaches double figures I'll be surprised.