But that is not done on the same runway at Heathrow. Never would a pilot be told to maintain his own separation from one ahead on the same runway. ATC apply wake turbulence separation at all times.
At times when both runways are in use for landing, usually early morning, an aircraft approaching one runway may be passed traffic information on one approaching the other runway ahead. When the second pilot has the first aircraft in sight the approach can be continued with no ATC separation between the two. Unless rules have changed wake turbulence separation is not required between aircraft on adjacent runways at Heathrow.