I think the problem here is that the "rules" as specified in CAP53 and CAP54 are illogical.
As aviators we are taught to follow the rules except when necessary to maintain safety. Unfortunately this is difficult when the rules are petty, illogical and plainly need changing.
The published rules (for SPA) plainly state that only a successful test (skill, reval or renewal) can be claimed as P1/S. However, for a very long time the general practise has been that a successful type or currency check is recorded as P1/S because the bod already has a license to fly the aeroplane and they just need someone there in case the skills are not what they should be (and the instructor is always shown as Captain in the tech log!)
Since the advent of JAR I've been getting people to record PU/T for a type conversion and P1 for a successful currency check as I think this fits within the spirit of the rules, helps the bod with their P1 time and is logically defensible.
In both of these cases I log it as instructional time because if the bod screws up and I fail to prevent the accident it is my license that's on the line and my name on the accident report. This means that 2 people are claiming P1 time but if the CAA wish me to do otherwise they can state that an accident during a currency check with an instructor will not be the instructors responsibility - or preferably change the damn silly P1/S rule.