I have now discovered that Sarah’ Amelia's daughter Emily Buchanan married her cousin, George’s Anthony's son Charles Johnson, on 26 September 1883 at the register office, St George Hanover Square (London).
Emily Buchanan, a 21 year old spinster, of 8 Peabody Avenue, Pimlico (London), was daughter of Walter Buchanan, cutter. Charles Johnson, a 21 year old bachelor and clerk (merchants), of 15 Westmoreland Street, Pimlico, was son of George Johnson, superintendent of police. The witnesses were Alfred Glassby, Edmund Wingrove.
Sadly, Charles Johnson, a merchant’s clerk aged 24, died at 23 New Street, Leominster, Herefordshire, on 28 October 1886 of phthisis (old name for tuberculosis). Perhaps he was in Leominster to recuperate away form the city, or perhaps he was just visiting. His death was registered by George Johnson of 23 New Street, father, present at the death.
More details at
https://birdsofcressingham.wordpress.com/2021/03/18/cousin-marriage-even-more-clues-on-mary-ann-johncey/