To answer my own question and update the thread - I have copied a response by the STA if anyone is interested.
The control of fire spread for buildings in-service is covered by the building regulations and the guidance in Approved Document B which, in the specific case, defines the Relevant Boundary as being the midpoint of roads. As you state, the principles are covered in BRE report BR187 and are primarily concerned with controlling fire spread between buildings for the intended service life of the building.
The control of fire spread during the construction phase of timber buildings is covered in the STA?s guidance document ? Design guide to separating distances during construction (see attached) and the reference distance is the fa?ade of any building in close proximity to the building elevation under construction. The focus of this document is life preservation and limiting damage to existing buildings close to the site boundary for the short period while the building is under construction until the in-service measures are in place.
The principles in BRE?s BR187 and the STA?s Design guide to separating distances are similar, but the reference points for calculating the relevant separation distances are different.
So the mid-point of the roadway is not the consideration point for safe separation distances during construction - only the distance to the actual neighbouring building despite the lack of external walls in the building under construction which would reduce radiated heat - doesn't seem logical to me!