Ooh good, another classic British murder!
I've really never been able to figure out how Wallace had the time to kill Julia before going out, as you point out, ColdCase. On the whole I think Parry more likely to be guilty than Wallace (there is at least a slight motive for Parry to attack and kill Mrs Wallace, while with Wallace himself there's nothing at all, no quarrels, no other woman, no substantial insurance policy payout). He and Julia lived a quiet and rather dull married life with no drama. All the same, the case for Parry as murderer is by no means open and shut either!