The CAA approve the RVA after consultation with the airport authority.
Without looking in the books, it's my understanding that the lower altitudes within a RVA are set in accordance with the highest obstacle in the sector, the premise being that providing radar contact with the aircraft is maintained the atco knows it's clear of the relavant obstacle.
If the atco is operating without radar (procedurally), the aircraft wouldn't be allowed to descend beneath the MSA, or if operating with radar it already has descended and subsequently radar contact is lost, it must climb to at least the MSA or an intermediate level above the MSA.
Hope that makes some sense.