Wake turbulence, which can never be reduced.
Interesting, is that an EGLL-ism? I (ADI & APS) would allow an IFR aircraft on a visual approach and maintaining their own separation from th eone in front to get closer to get closer than the wake turbulence distance.
The CAP493 says:
When a flight is operating visually (i.e. IFR or SVFR operating under the reduced separation in the vicinity of aerodromes, VFR, or IFR making a visual approach) and is following or crossing behind another aircraft, the pilot is to be informed of the recommended wake turbulence separation minima as listed at paragraph 9.5.1.
which I think means that the controller has passed the responsibility to the pilot and it's his lookout. It doesn't say you have to maintain the WT spacing in this case, nor does it specify any action if it is eroded.