When I left ATC at the beginning of this year, Heathrow was in the process of having MLS installed on all four ends of the parallels. As I recall, the principal user initially would be the BA airbus fleet. The advantage would arise in low visibility conditions when these aircraft could be positioned closer to the aircraft ahead because with MLS equipped aircraft, the localiser sensitive area constraints do not apply. This would increase hourly runway capacity in low vis conditions. I don't know how far the project has progressed.