Presumably you have seen the baptismal record for Letitia the daughter of George BARRATT, shoemaker and his wife Sarah @ St James, Paddington on 26 Nov 1815. There is also a marriage recorded between a Letitia Barrett & Richard Merritt on 28 December 1828 @ St Matthew Bethnal Green. This is clearly quite unconnected with your family, as Letitia Merritt appears with her husband & children living in Deptford in 1851 census (HO107; Piece: 1584; Folio: 497; Page: 14)and her death is recorded Greenwich in March quarter 1864 ref 1d p.513
The only reference in Ancestry to a Lydia Forbes is rather strange: 1841 census has a 25yr old girl of that name living in Portsea with a 2yr old child called John Moon; she is said to have been born "in foreign parts", and there is note of her being a seaman's wife, but that is crossed through. HO107; Piece 415; Book: 4; District: 15; Folio: 17; Page: 29
In Findmypast records I found the following: 01 March 1807 Lydia Forbes dau of William & Mary born 30/11/1806 in High street Whitechapel baptised St Mary, Whitechapel High Street - Entry No: 9690