St. Thomas here is the old St. Thomas Union of parishes - hence so many. I was born in the old workhouse for that union, though by then a maternity hospital.

Looking at the vol and page, this is 5b 70.
The birth coverage of St. Thomas for the Mar. quarter is from pages 28 - 70 and as the parish entries are alphabetically sorted, then this would suggest one of the later parishes with Withycombe or Woodbury being favourites.

The purchase of the certificate should confirm the birthplace