You'll have to ignore the Oneworld website - it's too buggy anyway. Here's what I'd suggest.

Post your list of desired destinations in the correct order here, and somebody will suggest an itinerary that complies with the Global Explorer rules. Since you're not planning to travel until July of 2025, you won't be able to book anything anyway, probably not until sometime around July or August of 2024, so there's LOTS of time to play with the route before you need to act on ticketing the trip.

And because the "cheaper" starting countries change from time to time, it's entirely possible that today's "cheap" origin points might become more expensive in that time, or some new "cheap" origin point emerges. Again, there's no way of knowing this far out.

In the meantime, you can look for a capable travel agent who can issue the ticket when it's time. There are some UK TAs, like Flight Centre, who advertise that they can book RTWs, so maybe that's a first stop. Their website - Round the World Flights & Tickets | Flight Centre UK - seems to focus on Star Alliance RTWs but I suspect they could manage a Oneworld RTW if they've got anybody skilled enough to put together a Star Alliance one. The phone call is cheap, anyway.

However as with all things related to booking RTW trips, it's essential that you know the answers before you ask the questions, so start doing your homework now so that you'll be ready to take command of your trip when it comes time to book it.