If you are acting under the authority of the commander whose name appears as such in your log book, and you are carrying out the functions of a pilot in command albeit under the commanders supervision, then you can log the time as P1/US. The time is recorded in the P1 column of your log book and should normally be countersigned in the remarks column by the aircraft commander.
That has always been my understanding of the rules, and is stated in the instructions for use at the front of my log book which is CAP 407 and as such a UK CAA publication. It doesn't matter which seat you sit in since the left seat occupant might not always be the aircraft commander. When 2 captains are flying together one will be the designated commander even though he may (and often does) sit in the right hand seat.