Hiya,
From the ANO Article 129 (Interpretation)
"‘Approach to landing’ means that portion of the flight of the aircraft, when
approaching to land, in which it is descending below a height of 1000 ft above the relevant specified decision height or minimum descent height;"
I realise this isn't an explanation but a pointer to the text that this definition comes from is the best I can do. I'm only at the stage of studying for my PPL airlaw so personally not too concerned about the IFR/IMC definitions and rules yet.
Splatt