You've got two questions here.
1. do people calculate minima from OCA/H etc or just use the figures off Jepps or equivalent? - everyone uses the figures off the plate since when you look in the relevant section you find that the figures on the plate are calculated exactly as you need to.
2. Do people add PEC to precision approach minima. Most light aircraft pilots don't, usually from sheer ignorance of what it's all about. In the heavier world, I can't remember the last time I flew an aircraft without ADC's that made the PEC = 0, so the problem goes away.
As for why it's there, it goes back to the amount of obstacle clearance you get during the approach and missed approach from PANS-OPS (so therefore it applies in most of the world, not just the UK). On a non-precision approach you have 250 feet (or more) clearance above obstacles so the PEC of 30-50 feet on most light aircraft is pretty academic. However, on a precision approach the clearance can be a lot less (theoretically down to the same 35ft you get on take off) so all of a sudden the height/altitude at which you start the go-around becomes much more critical.
Of course the problem is that there are always anomalies that make the system look silly, like Biggin Hill where the DA for the ILS if you add 50ft ends up higher than the MDA for the Localiser only approach
