I will speculate.
The UK information for healthcare professionals quotes:
The number of COVID-19 cases (2) in 660 participants ≥65 years old were too few to draw conclusions on efficacy
and
However, in this subpopulation, immunogenicity data are available, see below.
www.gov.uk
In other words, the trial proved efficacy in the overall population, but the approval depends on showing equivalent immune response (antibody generation) in older subjects rather than efficacy directly.
My guess is that the very small numbers in the older age group means there is a very large confidence interval on efficacy in that age group, and the lower limit of the confidence interval is 8%. If so, it absolutely does not mean efficacy is 8%, and the MHRA view is that the demonstration of equivalent immune response is sufficient to justify temporary authorisation.
As I said, just speculation.