This isn't really a problem, just annoying.

I'm with Virgin UK and the phone displays T-Mobile UK

How do I fix it???

Is there anything in c:\system\apps\phone\oplogo? Use, e.g., FExplorer from http://www.gosymbian.com/) to check. If so, delete the files in that directory/folder and restart the phone.

You can also use FExplore to create/set your own operator logo.