why no access to Three's network as Vodafone and Three is now one company?

I don't understand why Lebara UK customers have no access to Three's network as Vodafone and Three is now one company?
Smarty customers (they used to have Three's network only) now can use Three's and Vodafone's network also in the UK as I read the news.
From this comes my question why Lebara UK does not mak such an agreement with Vodafone that Lebara UK customers could also use not only Vodafone UK's, but Three UK's network also strengthening their signal significantly.

I assume eventually this will happen as they will be 1 network.