RTW tix are cheaper just about everywhere on the planet except US/CA/MX.

Often it's cheaper to buy a cheap OW to LON or BKK, (BKK's a safer bet due to their exchange rate remaining solid while the USD fell) do the RTW and set your home airport as a destination and just ditch the return to LON.

It's contingent on what you really want to see, ie see more of Asia and not as much of Europe/South Amer, do it from LON, want to see everywhere else but Asia, do it from BKK.

AKL is another cheap site for RTW tix but getting there from the US can be costly.