Again, round here this is basic knowledge for any scholar of aviation rules. And yes, we were also taught that the requirement can often be met by filing per R/T at first contact. Actually my R/T exam included such an example, for a VFR flight from EBAW to EBCI. One needed to be aware this call was actually an implicit filing of a flight plan, so some details needed to be given that wouldn't make much sense otherwise.
we have the situation where the written words from our regulator are not clear
That sounds quite plausible, I have no experience of the UK but similar words have come across from very various corners.