Japan is very low compared to UK and US. In business, from ITA Matrix...

HND-LHR on BA
Japan Passenger Service Facilities Charge International (SW)£14.60
Japan International Tourist Tax (TK)£4.90
Carrier-imposed surcharge (YQ)£123.60

LHR-HND
United Kingdom Air Passenger Duty APD (GB)£224.00
United Kingdom Passenger Service Charge Departures (UB)£51.72
Carrier-imposed surcharge (YQ)£176.50

JFK-LHR
United States Passenger Civil Aviation Security Service Fee (AY)£4.20
US International Departure Tax (US)£17.10
US Passenger Facility Charge (XF)£3.40
Carrier-imposed surcharge (YQ)£763.90

The places which may be cheaper with two one ways are
Japan, Hong Kong, Brazil, South Korea (no BA flights) and New Zealand (no BA flights)
Due to a combination of national taxes and BA imposed YQ.
But you can always check the difference using ITA Matrix if you are interested.
And you will find that starting in places in Europe like Ireland is much cheaper, but you can't use the voucher.

Depending on where you are flying. It can be cheaper to just buy J tickets for cash (possibly with another airline) than to fly BA on Avios, even using the voucher (if you value Avios at 1p each). It is the fact that this is very complex to work out and many people make mistakes in calculating value, that means the program is still economic for BA.
If everyone was hitting it for maximum value, they would probably have to do another devaluation.