It used to be, maybe still is, the case that LHR could apply an RVR barrier in low vis ops i.e. only traffic able to land below a certain IRVR would be accepted.
Not seen that done for a while though.
It was probably done when there were more a/c about that couldn't land in IRVRs below around 600m. Didn't want the stacks filling with traffic that couldn't make an approach.
Now that the bulk of traffic can land in very low IRVRs the low vis flow control takes care of the airborne holding situation (most of the time anyway).