First, BA also flies FLL-LGW, with comparatively low fares at that.
Second, DL recently flew MFW-LON, and pulled out the instant they were able to do so without losing the slot rights. No way that they're going to jump on that again this recently since.
Delta has never even flown to MFW (Magaruque, Mozambique) let alone MFW-LON. Not sure where you're getting that info or that they dropped the route there and maintained slots to LHR from it.
Get a grip dude.
MFW was an unofficial designator for South Florida Metro, originating from the amalgamation of Miami + Ft. Lauderdale + (West) Palm Beach; but no longer used by the IATA.
Even after Mozambique got the designation, most software would differentiate the two based on the hemisphere of origin. Even GCM did so until recently.
That, and context.
I, and others, have used that here for years with no problem in other posters understanding to whom it referred.
I had never heard MFW for MIA/FLL/PBI either. I also had to look it up and was reminded a loud bar/restaurant on PCH in Laguna is actually named after a country in Africa (because it's just an average (at best) steakhouse with spicy steak sauce and live bands lol). I thought maybe you made a typo but then since DL flies to some weird places in Africa I thought ok, perhaps they did fly there one-stop via LON for a brief period and I wasn't paying attention.
Since I was really curious why I had never heard it before, I actually searched for MFW on the forums and it's not as widely used and known as you say. In fact, in the past 5+ years it has been used here (6 pages on a search!), 100% of the time it has been used by you, someone quoting you or someone asking or arguing with you about it. Going back further, 2travel2know (whoever that is) used it once in 2011 or 2012, then after that 2 guys, who are also from Louisiana (ConcordeBoy and MAH4546) used it often. One of them wrote in a tone very similar to yours (MAH4546 I believe) and the only other poster I can tell who they are by what they type without reading their name is Jplatts (with him 100% accuracy, you not so much). Interestingly both of them joined 17 years ago (maybe that's when a.net started though) but they quoted each other often so I doubt they are the same person. Going back 12+ years there were a couple of other users who used it too. Someone else used it but it was a typo for MFE (McAllen, TX) and several people used MFW as acronyms for "my face when" and "max fuel weight". Almost every time someone used it as a city code, someone asked what it was for.
Also, why didn't you just state what airport DL flew from in South Florida to London from recently? I would love to know. Was it more than one?