Amazon has
just announced that the 70% option is being extended to the UK Amazon store. The UK price must be between £1.49 and £6.99. (NB this is lower than the US minimum price of $2.99, which is about £1.85 at current exchange rates)
http://dtp.amazon.co.uk/ is also now a valid URL, but just redirects to the .com address.
As far as I can see, if you've already selected the 70% option for a title available in the US store, and it's available to the UK, then the 70% option automatically applies.
Good news!