The 1851 census indicates you should be looking in the county of Northamptonshire on the 1841 census for the family
1851 census HO107 667 folio 27
Willow Street Oswestry Shropshire
Philip Keddie 38 Scotland Head Married platelayer
Maria Keddie 36 Hitchen Hertfordshire Wife Married
Philip Keddie 16 Chelsea, Middlesex, Son Rail lab
John Keddie 14? Watford? Hertfordshire Son
Robert Keddie 10 Ayston?? (difficult to read and probably spelt phonetically because it looks like Asaton so I can't be sure of the place name - there are other places it could bee.g. Ashton and Aston ), Northamptonshire, Son
James Keddie 7 Linslade Buckinghamshire, Son
Elizabeth Keddie 5 Gresford, Denbighshire, Wales Daughter
Mary Keddie 2 Gresford, Denbighshire, Wales, Daughter
George Keddie 2mths Oswestry, Shropshire, Son
It might be worth trying to find Robert's birth registration on 1837online to try and pinpoint exactly where he was born.
As the family moved around so much it will also help you if you had an old county map of England and Wales so you can see where the counties are, so you know which board to place your requests on. Chelsea though once in Middlesex was absorbed into London long ago and is now thought of as part of the West End of London. Hitchen Maria's birthplace (on the 1861 census as well) is in the county of Hertfordshire not Buckinghamshire.
According to Pallots marriage index
Philip Keddie married Maria Pyne in 1834 in Chelsea
Regards
Valda