I think that if the family story is true, it probably relates to David Senior.
I think the adoption of the name Kirby may have a completely different reason behind it and comes from Louisa's side of the family. I looked at the 1871 census -
1871 - RG10/4617/50/92.
12 Scarbro' Row, Normanton, Yorkshire.
David E Kirby, Head, M, 39, Labourer, b. Stepney, Middx.
Lucy Kirby, Wife, M, 29, b. Stepney, Middx.
Mary A Kirby, Dau, 3, b. Stepney, Middx.
Charlotte Kirby, Dau, 2, b. Stepney, Middx.
Clara Kirby, Dau, 6mths, b. Normanton, Yorks.
James Wm Bolden, Brother-in-law, S, 26, Sawyer, b. Stepney, Middx.
William Wood, Lodger, M, 33, Labourer, b. Wednesbury, Staffs.
Alfred Wigglesworth, Son, 2, b. Woodhouse, Yorks.
I couldn't find Lucy/Louisa in the 1861 so I had a look for James Wm Bolden, her brother. In the two earlier censuses he is living with his aunt and uncle, Thomas and Mary Kirby.
1861 - RG9/20/13/20.
15 Queen's Buildings, Brompton, Kensington, Middx.
Thomas Kirby, Head, M, 60, Cheesemonger, b. Wantage, Berks.
Mary Kirby, Wife, M, 55, Assisting in Business, b. Deal, Kent.
James William Bolden, Nephew, S, 18, Assisting in Business, b. Stepney.
Obidiah Freeman, S, 33, Assisting in Business, b. Debden, Essex.
1851 - HO107/1469/131/13.
15 Queen's Buildings, Brompton, Kensington, Middx.
Thomas Kirby, Head, 51, Cheesemonger, b. Wantage, Berks.
Mary Kirby, Wife, M, 45, b. Deal, Kent.
John Kirby, Brother, 70?, Cheesemonger, b. Wantage, Berks.
Matilda Kirby, Niece, S, 30, Saleswoman at ****?, b. Wantage, Berks.
William Bolden, Nephew, 13, Shop Boy, b. Sunderland.
James W Bolden, Nephew, 7, Scholar at Home, b. Stepney.
Charles Brown, Servant, S, 16, Shop Boy, b. Not Known.
Could it be that Thomas and Mary didn't have children of their own and wanted the Kirby surname to be passed on? It would be interesting to see if Thomas left a will and who benefited from it.
Have you found Louisa in the the 1861? I'd be interested to see it if you have.
Monica