Replacing windows like for like in a standard house is normally exempt as improvement work, but changing the size of openings, altering the appearance materially, or any work to a protected structure or in an ACA can require permission.
The planning test is whether the works materially affect the external appearance so as to render it inconsistent with the character of the structure or neighbouring structures. Swapping tired uPVC for new uPVC of the same pattern will not usually cross that line.
Replacing traditional timber sliding sash windows with uPVC casements on a period house is a different matter, particularly in an architectural conservation area, where the local authority may have adopted specific policies on window replacement.