Simply be honest:
"A single bedroom was converted into two separate bedrooms in 2014. The work solely involved changes to stud walls. At the same time a separate bathroom and WC were combined into a single room.
My understanding was, and remains, that neither planning permission nor building regulation approval was required for either of these modifications".
[NB: It's always best to refer to 'my understanding', rather than directly saying 'permission was not required', because if it turns out that you actually should have sought permission you can't then be accused of lying]