You are adding a rating to an FAA license. FAA rules apply, the CAA have nothing to do or say here.
They have if the hours are to count towards a JAA licence or rating. It is hours as PIC that are required for licence issue, not P1 (whatever that is). The definition of Pilot in Command, according to the ANO, is
"a person who for the time being is in charge of the piloting of an aircraft without being under the direction of any other pilot in the aircraft", it has nothing at all to do with being the sole manipulator of the controls. It is not unusual to be PIC of an aircraft and never touch the controls.
It is the CAA's view that if a safety pilot is required to be carried then the flight is being conducted under his/her direction and so the time does not meet the definition of PIC and cannot be counted towards the minimum required for licensing purposes. Plenty of people have been caught out by this in the past and have had their licence applications rejected until they have obtained extra PIC experience to meet the required minima, which is why you got such an immediate answer from Gatwick.