I did wonder about Surrey but I think all London was down as being in a county.
Might try the London Metropolitan Archive catalogue to see what range they have as that is probably the source of Ancestry's data.
It seems to be quite common for Ancestry to release part of a new dataset with the rest following later.
I usually search for SMITh as well! Though often Ancestry will let you search with a blank field or just a single wildcard.