No mmn is usually a sign of illegitimacy, because it means there is no "formerly" surname for the mother on the registration. It is not a 100% guarantee though, as nothing applies 100% and sometimes details were recorded otherwise than you would expect.
Simon's birth registration is an example of this - as you say, one would expect him to be registered either as Hannay (illegitimate) or untruthfully as Radford mmn Hannay if the parents presented as though already married.
But it was 1839, in the early period of a civil registration system first introduced in mid-1837, so neither the registrars nor the public had really settled into it yet. Incidentally it is in these early settling-in years that one very occasionally finds time of birth recorded for singleton births, but as practice developed this was only continued for twins and other multiple births as it might be important for inheritance purposes to know the order of birth.
Obviously you would need Simon's birth certificate in order to find out exactly how it was registered.
