A safety pilot operating on a UK licence can only log flight time if they have to assume control of the aircraft.
In the case of IF training you are there as a lookout, not a crew member and cannot log the flight. In the case of a pilot with a restricted medical requiring another pilot to be carried, again you can only log the flight time if the other pilot becomes incapacitated and you have to take over.
You can only log P2 in an aircraft certified for two pilots or where JAR-OPS requires a second pilot to be carried for public transport operations.