Yes, I suppose that is a possibility, but they did tell me in the shop that Orange was the roaming partner, and it also said that on the website.
Also the helpline (a mere snip at £8.00 for 45 mins to find out why I couldn't get a connection) said that it used to be orange but wasn't now.
If they had done 3G widely, as their name suggests, then they'd be fine. As it is the roaming agreements are a key part of their service and I don't think they should change them without informing their users.