I think it would work out better with UHT cream and non-UHT milk, sorry to say.
I am not an expert, and I'd wait for Linuxboy for a definitive answer, but this is what I'm thinking:
If you were to categorize whole milk as a mixture of milk and cream, rennet would react with molecules in the milk portion, not the cream portion. While the cream has milk in it, it is mainly fat, which is very important for flavor and texture, but it is the composition of the milk that allows for the structure of a cheese.
While UHT milk and non-UHT cream would be better than UHT milk alone, I'm not sure it would work much better for rennet cheeses.
What do you think LB? Am I close?