London is the capital city of England. It is in the South East region of England, geographically. London is split into quadrants; North, East, South and West. ‘Central London’ means the very middle bit, also often referred to as ‘Zone 1’ (that’s to do with the tube/subway). It is generally, most expensive to live in Zone 1. In each quadrant there are little neighbourhoods; Fulham, where Caroline lives is in southwest London (so spans the edges of South and West quadrants) and is Zone 2, I think, so not actually ‘central London’ but a very nice convenient location nonetheless.
‘Countryside’ is rural areas outside of cities and towns.
Ruth’s Georgian house is in Frome, in Somerset which is in the South West of England. It is actually further south than London (though not by much) and is much further West (by UK standards).
Does that help?!