spikeair - RW 23 at LHR is used when the southerly cross-wind component across the parallels exceeds 25 knots.
It is available for use by any type of aircraft when landing but only propeller-driven aircraft of less than 24 tonnes may use it for departure. That equates to something of the order of BAe ATP size.
It used to be used much more frequently before Terminal 4 was built as it was somewhat longer and had an ILS serving it. The required cross-wind component then was only 15 knots across the parallels.
As its use these days complicates the operation of the airport and reduces stand capacity the wind limit has been set so that it is brought into use only when really required.