The same LVP criteria as Manchester are applied at Heathrow, but there is no official differentiation between LVP "cloud" and LVP "visibility." The requirement is for the aircraft ahead to have cleared the Localiser Sensitive Area before the next aircraft has reached 1 nm from touchdown. Landing clearance should be given by 2 nm from touchdown and MUST be given by 1 nm from touchdown, or a go-around must be initiated. 6 nm spacing is recommended to achieve this.
In foggy conditions, cloud and visibility tend to fluctuate markedly, so I don't believe there is much to be gained by relying on pilots' reports which may be valid for extremely short periods. I suggest that if a pilot in the aircraft behind was relying on the pilot ahead reporting the weather conditions accurately, and if there was a catastrophe because of this, then those involved in the reporting extremely difficult position. At least transmissometer and meteorological reports can be tracked and recorded.
We are either in LVPs or not in LVPs, there is no middle ground. However, if the Air Controller observes that aircraft are vacating the LSA promptly, then the spacing requirements may be reduced accordingly. There is always a dialogue between Approach and the Tower under these circumstances. It is in everyone's interests to reduce the delays.