No birth registration or index found.
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Baptism register image, thanks FindMyPast, transcript by me.

“                         Page 35
“BAPTISMS solemnized in St Peter’s church in the Parish of Croydon
“  in the County of Surrey in the Year 1856
“. . . .
When Baptized:       “1856 Sepr. 4th
No:                  “276
Child’s Forename:    “Helen Rosalind, Dr. of
Parents’ Forenames:  “Alexander Copse & Elizabeth Henrietta
Parents’ Surname:    “Campbell
Abode:               “The Elms, Coombe Lane
Profession:          “Gentleman
Celebrant:           “J Drummond, Offs. Minr.
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In the 1891 census she and her husband were lodging in Grand Parade, Hastings, Sussex while their two children were with her mother in Paddington:

First name(s)  Last name  Role  Marital status  Sex  Age  Birth year  Occupation  Birth place
Thomas C  Williams  Lodger  M  M  37  1854  Barister at law   Stoke Newington, Middlesex
Helen R   Williams  Lodger  M  F  34  1857  -                 Croydon, Sussex

Ref:
Archive reference  RG12
Piece number       764
Folio              140
Page               4
Combined:          1891 ref: RG12, piece 764, folio 140, p. 4
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Not found from 1891 to 1939 but she reappeared in the 1939 register (not sure about the 1921 census) at 114 Inverness Terrace, Bayswater, London (which was her husband’s address and place of death on his probate):

                                                          Marital
First name(s)  Last name(s)  Birth date  Sex  Occupation  status  Schedule  Schedule sub number
Helen R      Williams  24 Jul 1856  F  No Occupation  W  228  1
Gwendolen M  Williams  20 Oct 1884  F  No Occupation  S  228  2
Joan V H     Williams  28 Apr 1886  F  No Occupation  S  228  3

Ref:            1939 ref: RG101/0411J/014/22

In fact she reappeared in 1932 as an executor of her husband’s will.

Further the above Joan V H Williams appears to be a new daughter for Helen, not seen either with her siblings at their grandparents in 1891 nor with her parents in Hastings in the same census.  She is to be found on the online GRO register, see below.
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Helen and Thomas’ children are definitely, from the online GRO index:

Name:                            Mother's Maiden Surname:
   WILLIAMS, ORLANDO CYPRIAN     CAMPBELL  
GRO Reference: 1883  J Quarter in MARYLEBONE  Volume 01A  Page 630
   WILLIAMS, GWENDOLEN META      CAMPBELL  
GRO Reference: 1884  D Quarter in MARYLEBONE  Volume 01A  Page 610
   WILLIAMS, JOAN VIOLET HELEN   CAMPBELL  
GRO Reference: 1886  J Quarter in MARYLEBONE  Volume 01A  Page 614

It would be good to find Joan Violet Helen in the 1891 census…  She is certainly to be found in 1921 in Staffs.  And the whole, but without sons, family showed up in 1911 at Taynton House, Burford Oxon.

First name(s)  Last name Role  Marital status  Sex  Age  Birth year  Occupation  Birth place
T Cyprian      Williams  Head  M  M  57  1854  Barrister at law  Stoke Newington Middlesex
Holm Rosalind  Williams  Wife  M  F  54  1857  -                 Croydon
Gwendolen      Williams  Daur  S  F  26  1885  -                 Marylebone London
Joan           Williams  Daur  S  F  24  1887  -                 Marylebone London
Violet         Coleombe  Serv  S  F  19  1892  Domestic servant  Framlode Gloucester

Ref:           1911 ref: RG14  PN8231 RG78  PN421 RD156 SD4 ED2 SN89

On the handwritten form, Helen’s name was definitely just that though it could be read as Holm.  Helen also reported that she had had three children, all of whom were then living.  Helen was then 54 so it sounds like the above list of three is spot on right and no further investigation is needed.
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Her death index:

Name:                          Age at Death (in years):  
WILLIAMS, HELEN ROSALIND       83  
GRO Reference: 1939  D Quarter in PADDINGTON  Volume 01A  Page 37
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Her Probate index:

1940
WILLIAMS Helen Rosalind of 114 Inverness-terrace Bayswater London widow died 27 December 1939  Probate London 14 May to Orlando Cyprian Williams house of commons clerk.  Effects £1082 13s. 5d.

I suppose I ought to get this will…
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The first bonus is that we learn what Orlando had become.

The marriage image, thanks Ancestry, from the parish register, transcript by me:

>                  Westminster  >  Christ Church, Lancaster Gate  >  1856-1885

“                                 Page 206
“. . . .
“1882. Marriage solemnized at Christ Church in the Parish of Paddington
“  in the County of Middlesex
No:            “412
When Married:  “July 13th 1882

                   His                   Hers
                   ===                   ====
Name:          “Thomas Cyprian Williams  Helen Rosalind Campbell
Age:           “full                     full
Condition:     “Bachelor                 Spinster
Profession:    “Barrister                -
Residence:     “MaryLeBone               68 Lancaster Gate
Father:        “Joshua Williams (dec)    Alexander Copps [ sic ] Campbell
Father’s job:  “Queen’s Counsel          Gentleman

“Married in Christ Church according to the Rites and Ceremonies of the
“  Established Church after Banns by me, E. Dorer

“The Marriage was solemnized between us, Thomas Cyprian Williams, Helen Rosalind Campbell
“  in the Presence of us, acampbell,  Francis E James.
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Their marriage index:

First name(s)     Helen Rosalind
Last name         Campbell
Marriage quarter  3
Marriage year     1882
Marriage Finder   Helen Rosalind Campbell married one of these people
                  William Howel Scrutton, William Howel Scratton, Thomas Cyprian Williams
District          Kensington
County            London
Country           England
Volume            1A
Page number       204
Record set        England & Wales Marriages 1837-2005
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