Royston,
You must be taking the mickey! 460' = 140m. For the 2700 m to work you would need to be on about a 55 degree approach with no lights.
If it is a 3 degree approach, Cos 3 degrees = 1.001, there is cock all difference and any difference is flown whilst the auto-call out says "minimums"
There are 2 issues with slant range! Flying over the airport you get a false sense of security looking straight down through a thin layer of fog, 1500m is 5000' near as dammit so from 5 k you think things are better than they are as you fly past looking down through a 100m layer. The other snag is that Vis can be affected by very low-level inversions occasionally.
But don't bother with slant range and Pythagoras because cunningly approach lights rarely begin at zero DME and extend about 900m short at most places so even on minimums you'll normally get in. That is why at 200 feet which is actually 1/2 nm from the threshold you are nearly over the approach lighting array and can therefore get away with 550m RVR rather than 1000m.