Not sure what the policy is at LHR. But I can tell you this:
The two parallel runways at LHR are more than 4300 ft apart. This allows both runways to be used completely independently of each other. In other words, one might consider these two runways as two different airports from an ATC point of view. As long as seperation is maintained between aircraft, considering SIDs and STARs [departure and arrival streams], it doesn't really matter to/from what direction the runways are used. Off course, wind condition minima must be respected at all times.
Therefore it is possible to have ie westerly bound take-offs on the northern runway, and approaches from the west on the southern runway. Now, I've never been to LHR, so I don't know if they actually do this, but it is certainly possible and legal. However it is not common practice since it severly limits the number of available approch and outbound patterns [STAR/SID], and generally creates severe headaches at Approach/Departure ATC!
At AMS, when tailwind component is less than 5 kts, aircraft can take-off and land with tailwind component. I've seen tailwind operations up to 10 kts tailwind component. The reason that they do this is noise abatement. They have the aircraft flying from and to the runways in such a way that produces the least amount of noise foot print in relation to the build up areas.
A couple of days ago, I was at AMS and both 01/19 runways were in use for take-off. 01L was used for take-offs to the north, 19L used for take-offs to the south [27 was used for inbound traffic, since 06/24 was unserviceable].
I've even seen aircraft arriving at 06 and 27 simultaneously [with departures at 01L]! Now if you take a close look at AMS runway lay out, you'll notice that if an aircraft landing on 06 must make a go-around, it will meet conflicting inbound traffic at 27 head-on!! Furthermore, if an aircraft has a go-around at 27, it will cross conflicting traffic departing from 01L!
However this type of operation is only permitted in good VFR condition, with cloud base at least 2000ft or so. But it does show that some creative runway combinations can be used.
So for LHR, as long as tailwind component is under 5 kts [or upto 10 kts depending on runway conditions], all runways can be used to/from all directions.
Hope this helps.
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