I believe your pilot mate can hand over control to you if he thinks you competent with no visit from the CAA, though Insurance may be another issue.
Basically you can't log it, unless you are directed to fly by the Queen, then you can fly whatever you [she] like[s], where ever you like without breaking a single rule
As an FI mate of mine says, the Logbook is a legal document. Never write anything in there that will incriminate you....[like....nah, better not go there
]
Cyer
EA