I think you are getting two different things confused here.
Black rings - these were only ever on some 757s and 737s. The 737-400s are all gone, I think the last went last September. I think the ring was something to do with electrical conduction or lightening protection, but with more modern aircraft it is possible to achieve without the ring,
MLS - this is a “new gen” instrument landing system, like an ILS v 2.0 I suppose. I don’t believe any BA 757s or 737s ever had it. MLS is fitted to virtually all the LHR based A320 series aircraft and LHR’s runways are all MLS capable so BA A320 series aircraft coming in to LHR all use MLS. Obviously none of the BA A320 series aircraft have the black ring so clearly MLS and black rings are not linked.