Some movies are available to rent in HD but only available to by in SD. why is this if the HD copy exists surely we should be able to buy it.
Because that's the way the owners of the rights licensed the movie. Probably they think they'll make more money selling you the Blu-Ray disk, which often includes a lot of extra cruft and hence costs more.
It may also be available to buy in HD in a few months. Just keep checking the movie every week and be ready to buy it when it is available.
I've seen movies that were for rent for a few years suddenly become available for sale in HD recently.
Retrieving data ...