A "sitting" should be sufficient to sit all the exams in one go
That's rather sensible and logical (and complies with EASA).
Unfortunately it is not law as EASA says a "Sitting" is a period of time
for a candidate to take an "Examination" not for them to take
an "Entire set of examinations".
Maybe CAA thought one (very long for the candidate) day was sufficient
to take all PPL exams?
Maybe they thought it didn't matter if a PPL candidate, who wanted to
take all their exams "in one go" used up 2 or 3 sittings - as it still left at
least 3 more should they not be successful in any paper(s)?
Maybe they haven't considered it at all and a Ground Examiner should suggest it to them?