I've been wondering if it's possible to hack them. No doubt eventually some will try.
People will try.
That's one of the reasons SMETS1 meters can't all be upgraded to work with current systems - they don't have the encryption capability to talk to the secure network. Apparently GCHQ were involved in the security design - after the original security was found to be "not robust". Some SMETS1 meters can be upgraded with a new comms module, many will need to be replaced - so much for early adopters of smart meters. And I believe SMETS1 meters were still being installed into last year.
Of course, we are expected to believe that this massive system, with something in the order of 60 million devices (almost all lacking physical security) connected to one end, and many suppliers all connected to the other end, and many meter operators involved, and god knows how much information - will be 100% secure for all time. Yeah right
I suppose it will probably be "secure enough", but that's not really an excuse for storing something in excess of 50k records per customer (48 records/day * 2 meters * 365 days/yr * a couple of years ) - so over 2,000,000,000,000 records in one big database. What could possibly go wrong