Having posted the question I was very interested in everyones reply. At the airport I had in mind there is a railway line that crosses the approach. A landing before the threshold from a 3d glideslope would result in passing close to any trains on the line. I agree with others sentiments that a 3d approach, or flying the PAPIs, in most singles would result in it being impossible to make the threshold in the average GA single after an engine failure. What you decide to do is of course up to you and your risk assessment of an engine failure on the approach! Flying a much steeper glideslope may mean a landing before the threshold with solid clearance of the railway line in this case entirely feasible, although why you would perhaps want to land before the threshold is another issue. Part of my question was whether it was legal to do so, and whether in fact the CAA would take any action - I am still not certain of the answer. As a by the by I have noticed at some other airports with displaced thresholds with no obvious reason (I am sure there is one, just not apparent to me) the pre-threshold bit is sometimes in poor condition which for other reasons may also make it unwise to land pre-threshold. I agree that the rules would change IFR.