The Hundred of St Briavels usually means The Forest of Dean but in Parish Records particularly of this date it often means that the person did not live within a defined parish. Another term used is "extra-parochial" land.

If we look at the 1851 Census we see that the family is living in Ellwood. Here is the entry:
House Number Given Name Surname Relationship Age Estimated Birth Year Gender Birth Place
44 Richard Powell Head 59 1792 Male West Dean, Gloucestershire, England
44 Mary Ann Powell Wife 57 1794 Female Newland, Gloucestershire, England
44 Henry Powell Son 28 1823 Male West Dean, Gloucestershire, England
44 Isaiah Powell Son 17 1834 Male West Dean, Gloucestershire, England

Richard's age tells that he was born c 1792 and we find this baptism in 1791:

Year: 1791
Month: May
Day: 8
Parents_Surname: POWEL
Child_Forenames: Richard
Fathers_Forenames: Richard
Mothers_Forenames: Elizabeth
Mothers_Surname:
Residence: Ellwood
Occupation:
Officiating_Minister:
Event: Baptism
Memoranda:
Notes: Any abbreviated forenames have been expanded to the full name
Register_Reference: P227 IN 1/4
Page_Number: 803
Parish_Chapel: Newland

Mike

Richard & Elizabeth POWEL(L) - appear to have a Son Thomas christened 1786 - residence Ellwood.

Fod Records

Burial at Newland - 14 December 1829 - Richard POWELL - residence Ellwood - age at death: 70 Years

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I think this is his Will on Ancestry dated 23 January 1837 - as the Will appears to be annexed

Adcorn? cum? Testo annexed granted to Richard POWELL one of the sons and next of Kin

Sub: £20

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This is the last Will and Testament of me Richard POWELL of Ellwood in the Forest of Dean and Hundred of Saint Braivels in the County of Gloucester.

I give, devise, and bequeath, unto my son Thomas Powell my dwelling house and orchard adjoining thereunto, likewise my largest piece of Land known by the name of Great Patch and after his decease to his lawfully begotten Son and Heir Richard POWELL.

I also give and bequeath to my Son Thomas POWELL the garden in front of his house to be at his own disposal and I direct that my son Thomas POWELL shall pay to my dear wife Elizabeth POWELL the sum of Three Shillings and Sixpence weekly.

I give, devise and bequeath unto my Son Richard POWELL a piece of land known by the name of the Little Patch to be at his own disposal after the Decease of my said Wife.

I give and bequeath unto my Daughter, Sarah ROFSER a portion of my Great Patch on the North side of the same from the corner of the hedge to the Ash Tree opposite to be at her own disposal.

I also devise my Son Thomas POWELL to pay to my Grand Daughter Elizabeth TAYLOR the sum of Five pounds of Lawful Money within one year after my decease - and when she hath attained to the age of Tweny One - And I hereby revoke all my former Wills - In Witness whereof I have to this my last Will and Testament contained in this paper, set my hand and seal this Twenty Fourth Day of October in the year of our Lord One Thousand Eight Hundred and Twenty Five.

The mark of X Richard POWELL

Signed, Sealed - published and declared by Richard POWELL, the Testator, as and for his last Will and Testament in the presence of us who in …. presence at his request and in the presence of each other have hereunto subscribed our names as Witnesfness thereto.

Henry POOLE - Cler,
Benjamin TAYLER
James JONES