Isn't the RVR take off requirement for commercial ops the same as the RVR detailed on the approach plate at that airfield
No.
The ANO is fairly loose on the subject, placing the responsibility in the hands of the operator. PT Ops Manuals, in order to be approved, will contain two stipulations:
One has to do with the lighting on the runway, and the acceptable vis. For Keef's 150m you need centreline lights for example. An RVR of around 175-250m would be typical for a Biggin or Southend, from memory.
The other has to do with getting back on the ground in the event of an emergency. Typically you have to ensure that there is somewhere above minima (and forecast to remain so) within 30 mins flying time, flying time being calculated on the basis of an engine failure.
Will