Microwave landing system (MLS) at Heathrow

[G-CIVA]

I have never used it, and never heard it mentioned technically. The switch is there, though, as you say.
Maybe it makes the wings fall off...

From the original fleet of 57 B747-436 aircraft delivered to BA they have 37 still in service. 

According to the reference material the ILS/MLS Switch was fitted to the earlier production models but placarded INOP .... it probably succumbed to the cost vs benefit analysis as other have mentioned.

The ILS/MLS switch is fitted to G-BNLK - NLY, G-CIVA - IVO

It is not fitted to G-CIVP, G-CIVR - IVZ, G-BYGA - YGG.

[Michael Benson]

What again was the reason for this, over ILS I mean? The smaller distance to clear the LSZ?

Yes, aircraft only needed to vacate 86m rather than 137m which means rather than 6nm spacing between pairs we can do 5nm.

The other advantage that arriving after a departing in the ILS world require the outbound to over overflown to LOC aerial (end of the runway) before the inbound was at 1 or 2nm.  In the MLS world the outbound just needs to be airborne - thus again saving 1nm. 

[Markus Vitzethum]

Cool - there is always a switch I haven't heard about before.

Indeed, it is shown on this image of G-CIVO, albeit very small.
http://www.airliners.net/photo/British-Airways/Boeing-747-436/1472007. However, I do not notice an INOP sticker near that switch

Or, from a closer location here on G-BNLF, also not INOP.
http://www.jetphotos.net/photo/7715598

however, not on this photo of G-BNLY:
http://www.airliners.net/photo/British-Airways/Boeing-747-436/1740851

From jetphotos.net, I see the switch installed on G-BNLF/LZ/LM/LN/LP/GF/   G-CIVH/VO/VN/VF/VG/ (at least at the time the photos were taken).

Markus

[G-CIVA]

However, I do not notice an INOP sticker near that switch

Where the reality of life often does not match up to the owner operator reference material:

[G-CIVA]

However, I do not notice an INOP sticker near that switch

There is the possibility that such stickers were not put in place or subsequently removed during MX; there is also the possibility that the panel in question ... P2-4 (in the case of 'NLY') may have been replaced altogether due to a defective component on one of the other switches during the aircraft's long service life.

I expect that the switch was present on panel P2-4 on delivery.