This is so.
Furthermore, the CEPT rules some years ago used to mandate the use of a /P suffix when operating abroad under the CEPT rules. This is no longer mandated under the current CEPT rules.
In the UK, the use of suffixes such as /P, /A, /M, etc is optional. However Ofcom's guidance notes now helpfully state that it is up to the individual countries to decide whether a suffix is mandatory, optional, or not allowed at all, when foreigners are operating on their soil!
"2.109 Under Note (d) to the Licence, a licensee may use the suffixes ‘/M’, ‘/P’, ‘/A’ or ‘/MM’, to denote a location away from the Main Station Address though their use is optional. If a licensee were unsure which suffix was most suitable or thought that using a suffix could affect the requirement for the station to be clearly identifiable at all times, it may be simpler (and would not breach Note (d)) to omit the suffix. That is not a choice with RSLs, as the Licence mandates their use.
2.110 Other administrations may not recognise UK call sign suffixes or permit their use. When operating overseas, therefore, licensees must comply with the requirements of the authorities on the use of suffixes."
And then there is the question of whether it is ever permissible for a UK Foundation or Intermediate Licence holder to operate overseas. It is not permitted under CEPT rules, but it might be possible to obtain permission from the foreign licensing administration, in which case it's none of Ofcom's business!