@anamariap already shared their experience and recommendation. I agree and it seems to be the general consensus on the topic discussing Sopra Steria topic (have a look https:british-citizenship/experiences-using-new-ukvcas-for-naturalisation-citizenship-applications-t269057-440.html ) ---
- upload as much as you can yourself;
- bring all documents to the centre at the appointment;
- they will scan what you couldn't (there may be an extra fee for this in future, but until recently there wasn't);
- they will always scan all valid passports (or at least one other proof of identity).
There are more useful details in the topic and other topics too on details of which appointments are free, which are paid and how much, what is available, locations, some teething problems, but getting better. So, have a read-through.