Do you have the 360 set up for 4K? Settings > Audio & Video > HDMI resolution
Are any onscreen messages displayed when trying to access the VM UHD channel (175 and 999) such as those indicated in the following post by VM Forum Staff member Steven_L?
Ultra HD & HD TV | Virgin Media Community - 5671953
Which actual TV make and model do you have?
For example Hisense 43E6QTUK, or Samsung UE40F6000FK
The BBC have a list of TV's that they support for 4K <Here>. Is your device on their list?
Are you using the correct HDMI port on your TV? The BT website have an article <here> that lists a large number of TV models by manufacturer and the ports that are 4K compliant (for example older Samsung's that use (MHL) and/or the Samsung Evolution kit