If I remember rightly from my IMC, it's because on a UK VFR chart terrain below 500ft is not marked and obstacles that are less than 300ft AGL are not always marked (some are).
So theoretically you could have a 495ft high hill with a 295ft high mast sat on it and neither would be marked on the chart. This gives you 790ft, add 1000ft to that, round it up and you get the 1800ft figure.
I think that's right! Happy to be corrected if not.
Edit: D'oh, beaten to it!!