Ok, the penny has finally dropped, I now understand. Thank you all so much for taking the time to explain it to me. Funnily enough the BA sales agents who have been trying to book this flight combination also don’t understand the open jaw ruling.

Not sure what I will do now, probably use the 2 4 1 on something else. I don’t want to buy a one way ticket MIA to LHR (ultimately this is all I really need to get back after our cruise) as BA seems to financially penalise you for doing this. I do have a couple of e-vouchers £400 total (flew Club World last month LHR to Haneda and BA failed to load ALL Business Class luggage. I do have Virgin points and reward vouchers with them, I will explore this option when MIA flight is in date range next week.

Once again, thank you.