In the UK LVPs are in force when:
The IRVR is less than 600m and/or
the cloud ceiling (BKN or OVC) is 200 feet or less.
There are differing categories of LVP for use by ATC and Airfield Operations, but to a pilot the term LVPs is all encompassing and any relevant info would be broadcast on the ATIS.
In the situation you describe, LVP "cloud" would have been in force. Inbound spacing would have been increased but not to the same extent had LVP "visibilty" been in force.
At Manchester in LVP cloud, all runway exits are available but in LVP vis. this is not the case.
A LHR ATCO will probably be able to tell you their specific requirements.
As for using pilot reports to base ATC procedures on, its a non-starter I'm afraid. We use the IRVR for the vis., a laser cloud base recorder for the cloud base (funnily enough!) and if thats all broke then the Met vis. and estimated cloud base from the trusty Met Observer.
Its all about protecting the ILS signal for following aircraft and that means slowing the traffic. You want to fly slower on approach too, so it all backs up. Landing rates may fall by 30-50% and delays go up.
Hope this starts to help a bit.
Merry Christmas