Where does this come from?
Your altimeter is a measuring instrument, which may have a systematic error that depends on conditions, often related to the siting of the static ports. Some airframe/altimeter combinations have zero PEC at approach speeds, others a few tens of feet in one direction or the other. 50 feet is a fairly conservative estimate, and should only be used if your POH/AFM doesn't offer a figure.
What minima do private pilots and GA pilots in the UK use? JAR-OPS or must they always calcualte their own like it shows in this manual using OCH and procedure minima?
Without checking in detail, I don't believe there's a difference between the requirements of the UK AIP (the legal requirement for non-public transport flights) and JAR-OPS1.
As BizJetJock suggests, it's a bit daft to add PEC for a precision approach but not for a non-precision approach, though that is the advice. In practice, for single pilot ops, while RVR is known to everyone, the altimeter reading at which you make a decision to land or go around is effectively a matter that remains between you and your priest.