Don't bother to ask the Royal Mail, Jason. I checked with my local USPS branch and was informed that as of August 10 (13 days ago), there is no longer ANY kind of postal money order they can sell that can be cashed in Great Britain. They don't know why, or whether this is temporary or permanent. There are still other countries where the International Postal Money Order is valid, just not Great Britain.

My guess is it's tied up with terrorism, money laundring, and all that nonsense. However, maybe JohnB's suggestion to use travelers' checks is the best.