From - Section D, Notes to the Licence
When Operating at locations other than the main station Address, it is recommended that the following suffixes be used,
II. If the Licensee operates the Radio Equipment at a Temporary Location, the Licensee may use the suffix “/P”.
It also covers, /A, /M and /MM.
So you are not breaking the terms of your licence by not using /P, as it is a recommendation.
Ofcom clarified this when they last updated the licence prior to that it was thought that the use of /P /M etc was a licence condition.
However the use of /P has become established practice and it does make it clear that your operating away from your Main Station Address.
For clarification on the air, most use the appropriate suffixes, and Sota is no exception.
I would suggest you print out a copy of the licence, if you have not already done so.