Here is my high level understanding of the UK smart meters.
There is currently two types of meters SMETS1 and SMETS2.
SMETS1 is being replaced since it only connects to the single supplier and requires meter change when you switch supplier.
SMETS2 sends the data to a Data Service Provider (In UK there is only one company to my understanding "CGI UK Limited") The suppliers then get a feed from this and also the Distribution Networks.
For 3rd party to get access they have to be a "Authorized Party" and have explicit conecent from the customer.
To my understanding the SMETS2 meters use Zigbee so you could get a feed in you home using this (Pretty sure to get paired you will still need to contact your energy supplier). You could also ask your Energy provider to give you access to the feed via a API but this will be delayed to my understanding.
This is very different and and they use Data Collectors. There are a few big ones so once you have agreements setup with these companies and the customer has agreed you can get access to the feed via one of their API's.