Most of my ancestors were also signing with an 'X' too. This family aren't in my ancestry unfortunately but I do remember meeting Louisa when she was being looked after by one of my aunts and uncles, she died aged 99, about three months short of her 100th birthday, the village were already planning for that event as never having occurred before there.
I suspect that David was given the Thornton's London House at 23 Finborough Road, Brompton as he is initially listed as housekeeper in 1871 then annuitant in 1881, he died in 1885 and in 1891 his wife was still there recorded as a Lodging House Keeper - can't see the Thornton's allowing the use of the house for such if they still wanted a London House for 'the season'. It's an impressive five storey terrace property clearly built to impress when new.
My cousin inherited the letter along with some of Louisa's jewellery when the aunt who had looked after her died. We've just researched the family because we both remember Louisa when she was an old lady and knew her former house well as another aunt and uncle lived there for best part of 30 years.
With everyone's help I'm amazed at how much I've found out about this family, David certainly travelled about a lot and seems to have been an ambitious guy, presumably must have learnt to speak and read/write French, perhaps a requirement of his employment initially in 1841 in Lord Carberry's household in Belgrave Square in London - a very 'posh' area at the time, then at the Thornton's household at Brockhall in Northamptonshire 1851 to 1861 where the head was the Deputy Lieutenant presumably of Northamptonshire.