Not sure where to post this or how new it is, so I am putting it here. There is now a Rent to Buy option in VOD. Some movies that you are currently renting can be purchased for only the difference between the two costs. This option had been requested at least twice in the Idea Exchange. But it's implementation was never noted.
This option can be found here:
On Demand > My Library > My Upgrade Offers
The offer details can be seen here:
On Demand > My Library > My Current Offers > Rent to Buy