Actually, they do have some online forms to calculate shipping. Though its no API you could build your own service to calculate shipping using a series of http request that dynamically fill in the form and retrieve the info. Perhaps a cron could be ran every six months and run an update so you didn't have to do everything manually. Not sure if something like that possible given your situation, but may be worth a try.
(you need to turn off javascript to view the non-flash version)
http://www.royalmail.com/portal/rm/c...diaId=63900706
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