If at the top of the birth certificate written after Registration District is Kingsbrigde Union that would be the registration district and the sub-district would be Modbury.
If the child was born in the Workhouse is would state this or similar wording with sometimes just an address of the Workhouse given. At the date you are looking at it usually gives Workhouse as the place of birth if that is where the child was born, it was only at a later date that only the address of an institution was used.
If the child was baptised you might find more information regarding an address in the baptism register as sometimes the baptism registers for this period give a place within a parish in the record.
The boundaries of these Poor Law Unions were used as the basis of Civil Registration Districts when they were introduced in 1837 by the Births and Deaths Registration Act of 1836. Thus District boundaries took no regard of county boundaries and as a result Registration Counties were created for statistical purposes. The boundaries of the unions would later be re-used to define rural sanitary districts in 1875 and yet again for rural districts in 1894.