Smonday

I agree that the UK is one of the cheapest places to begin a Oneworld Explorer- this only holds true for Economy Class. Remember though that you need to also consider the price of a ticket to get from the US to UK to begin your RTW. I trust it will still work out cheaper for you.
If you are in the US and want to originate in the UK, you can buy the ticket at UK prices. The agent will simply convert this into USD.

Yes, back-tracking is permitted WITHIN a continent. Backtracking is not permitted BETWEEN continents other than where the exceptions described by ExMo apply.

Salt.