No birth record found on FreeBMD.  In the 1901 census there is an Averil Powys who was a pupil in Reigate Surrey and born c. 1890 in Philadelphia, USA.  Her mother is unclear on this census.
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On the 1901 census she was at Royal Asylum Of St Anne's Society, Reigate, Surrey.  This was a charity for the children of parents who had come down, or occasionally up, in the world; it sounds to have been very strict.  Other details:

First name(s)                       Averil
Last name                           Powys
Relationship                        Pupil
Marital status                      Single
Sex                                 Female
Age                                 11
Birth year                          1890
Birth town                          -
Birth place                         -
Birth place other as transcribed:   N AMERICA PHILADELPHIA U S A
Birth county                        -
Occupation                          -

The reference was:
Archive reference  RG13
Piece number       628
Folio              114
Page               5
Combined:          1901 ref: RG13, piece 628, folio 114, p. 5
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Family Search has provided this abstract of her birth record, note that she started life as Marjorie:

Name           Marjorie F. Powys
Sex            Female
Birth Date     07 Mar 1890
Birthplace     Philadelphia, Pennsylvania, United States
Father's Name  Walter Powys
Mother's Name  Ada Powys
Record Number  130

Document Information
Reference ID                     128
GS Film Number                   1289329
Digital Folder Number            004015777
Image Number                     00202
Indexing Project (Batch) Number  I02452-1

Collection Information
Pennsylvania, Philadelphia City Births, 1860-1906
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Remarkably Ancestry.com produced a USA census form of April 18 1930 for Averil, her sister Evelyn and her sister’s husband Fred E Quipp, all living in one rented house on Sterling Road, Watchung Borough, Somerset, New Jersey, USA:

Name:                       Averil F Powys
Birth Year:                 abt 1892
Gender:                     Female
Race:                       White
Age in 1930:                38
Birthplace:                 Pennsylvania
Marital Status:             Single
Relation to Head of House:  Sister-In-Law
Home in 1930:               Watchung, Somerset, New Jersey, USA
Map of Home:                Watchung, Somerset, New Jersey
Street Address:             Sterling Road
Dwelling Number:            69
Family Number:              70
Attended School:            No
Able to Read and Write:     Yes
Father's Birthplace:        England
Mother's Birthplace:        England
Able to Speak English:      Yes
Occupation:                 Book Keeper
Industry:                   BANK
Class of Worker:            Wage or salary worker
Employment:                 Yes

Household Members  Age  Relationship
Fred E Quipp       67   Head
Evelyn M Quipp     47   Wife
Averil F Powys     38   Sister-In-Law
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Ancestry has also delivered a poor copy of the 1940 census in which Evelyn and Averil were still together but Evelyn was a widow, so Fred must have died.  I cannot read Evelyn’s occupation but Averil still seems to have been a bookkeeper at a bank.

The transcript by Ancestry is:

Name                               Averel Poweys
Respondent                         Yes
Age                                49
Estimated Birth Year               1891
Gender                             Female
Race                               White
Birthplace                         Pennsylvania
Marital Status                     Single
Relation to Head of House          Sister
Home in 1940                       Plainfield, Union, New Jersey
Map of Home in 1940                Plainfield,Union,New Jersey
Inferred Residence in 1935         Plainfield, Union, New Jersey
Residence in 1935                  Plainfield
Sheet Number                       63B
Occupation                         Bookkeeper
Industry                           Bank
Attended School or College         No
Highest Grade Completed            High School, 2nd year
Hours Worked Week Prior to Census  40
Class of Worker                    Wage or salary worker in private work
Weeks Worked in 1939               52
Income                             1235
Income Other Sources               No
Neighbors                          View others on page

Household members                  
Name                               Evelyn Grupp
Age                                58
Name                               Averel Poweys
Age                                49

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In 1955 may have retired as there is this entry on a Social Security Application and Claims index:

Detail
Name:        Averil Powys
Birth Date:  7 Mar 1890
Claim Date:  4 Mar 1955
SSN:         154015958
Notes:       26 Nov 1976: Name listed as AVERIL POWYS
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She appears to have died in Los Angeles, from this California death index:

Detail
Name:               Averil Powys
Social Security #:  573442350
Gender:             Female
Birth Date:         7 Mar 1890
Birth Place:        Pennsylvania
Death Date:         2 Apr 1971
Death Place:        Los Angeles
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