Certainly going back about 8-10 years ago we were advised that aircraft fly a much slower speed during Cat 2/3 approaches.
However subsequently that would appear to have changed and I have not encountered any aircraft advising that they were unable to fly anything but the normal approach speeds during LVPs.
As previously stated let us know what you require to do and we will accomodate you as best we can , but please don't do it when you are established . It takes time to work a sequence and to position aircraft behind you we need extra miles if you are going to reduce your speed substantially.
This again is why we need Fam flights, but both NATS managment who don't believe they are any benefit to ATCOs ., and Airline Companies who will not allow us up the front are doing absolutely nothing to re-establish them .