Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

"With Amex UK, I can transfer points to:

Emirates, Alitalia, British Airways, Etihad, Cathay Pacific, Delta, Finnair, Air France/KLM, Iberia, SAS, Singapore Airlines, and Virgin Atlantic; all are 1:1."

From that list, you need to investigate which -- if any -- of the FFPs over one-way redemptions with low or no surcharges for a LON-Japan trip. You can forget about redeeming BA Avios because of high surcharges, unless you redeem for Reward Flight Savers between LON and someplace like IST or LCA, and combine that with a paid ticket on another carrier.