The 60kt value comes from the regulations ops 1.405 has the following note:
Note: ‘Relevant’, in this context, means that part of the runway used during the high speed phase of the landing down to a speed of approximately 60 knots.
this is for commencement and continuation of approach.
1.430 deals with take off and here there is no value given to the definition relevant.
for low vis ops it simply states
Note 3:The reported RVR/Visibility value representative of the initial part of the take-off run can be replaced by pilot assessment.
Note 4: The required RVR value must be achieved for all of the relevant RVR reporting points with the exception given in Note 3 above.
I'm not sure how appropriate it is to use the 60kt value for take off. In any case values for accelerate stop distance are given to V=0 not V=<60 so there would be no way of checking relevant to < 60 kt.