What are the differences in requirements between a Non-Immigrant O Family visitor visa issued in London and a family visitor extension issued in the Changwattana Embassy in Bangkok?

Can I ask what's wedlock

How about you spell out to me what a "family visa" means to you?

To get a year extension inside the country at the immigration office based on marriage or supporting children you show 400K baht banked in a thai bank account (for marriage 2 months before you apply) OR you show proof of income in the form of an affidavit of income from your consulate OR if you're legally working here your tax statements showing 40K baht a month in income.

The real questions when trying for a yearly extension are:

-are you married to a thai national <- if NO you can't get a marriage visa

-if you have half thai children, did you have them IN or OUT of wedlock <- if you had them out of wedlock you as the father have no legal parental rights to the children (even if your name is on the birth certificate and they have a passport from your country) until you go thru family court and get recognized as the legal father and granted parental rights you can't use them as a means to get a visa or an extension INSIDE the country.