Hi Touch'n'oops,
Actually if you're acting as a safety pilot under FAA regulations then you can log the time legally as P1 as long as the other pilot is under the hood and you are in VMC conditions within an aircraft that you are rated to fly.
However I would certainly suggest you don't do this as although legitimate in the FAA world, the CAA could ask you how you gained the hours. If they did ask, how would you respond?
They (the CAA) probably wouldn't take too kindly if you say safety pilot time under FAA regulations.
I've read on a US based Pilot Forums that some US airlines are becomming wise to certain schools aircraft registrations and immediately halfing the number of P1 hours you have... however this is speculation so don't take my word for it.
When you're hour building in the States I would recommend keeping all receipts etc just in case.
Best wishes,
Charlie Zulu.