This 'is' an expensive journey.
If it were me, I'd start with the marriage record for Thomas and Harriet, see what that record has to say about their father's names and occupations and, from there, work out which birth registrations are most likely to be the right ones for each of them.
Help will be available at that stage to work out which ones may be the right ones.
It's a slow and step by step journey.
Boo