That's not how it works... A heating engineer will appraise and discuss with you before putting in a quote to do the work.
Please don't be tempted to call someone in on the pretence that they are quoting to get the job, only for you to be extracting advice from them.
Even though your builder is acting as a SPA for you, I would strongly recommend that you appoint your own heating engineer and even electrician because they will focus on your comfort and how you live in your home... I see it so many times where builders/project managers decide on customers' behalf and they are left wanting after handover.
Someone coming in to do the assessment before doing a quote seems like a lot of scope for being impartial. I'd rather pay someone to do a professional assessment knowing they have no personal incentive to suggest one thing over another. Not saying everyone is dishonest, but there's enough to make me sceptical of this.