Ant T.
Been doing a bit of searching. It appears the only way a large
download can be prevented is if you do not have enough memory on your phone/iPad to accommodate it.
So, as a workaround, you could load up your device with Tunes, Videos and Pictures.
Not satisfactory, I agree. Perhaps in a future update ...
If you want to provide feedback to Apple you can use :
Apple - Feedback
Don't expect an answer, but I have it on good authority that suggestions made here are considered.
Good Luck