If I end up with a OW Explorer fare, I'm thinking I may start it off in Cairo for the purpose of making the whole RTW in business class cost about £5000 instead of about £8000 ex-London. As I need to buy 3.5 tickets any saving helps a lot.

I've checked out getting to Cairo and this also solves my problem of getting to Addis Ababa. I can go on a separate ticket LHR->ADD->CAI one way (with about 10 days stopover) for about £580pp in economy.