A red 'X' could mean that someone's broken down in that lane, there are workmen in the lane or there's another obstruction. You must move out the lane if you see a red 'X' over it when you're driving. Don't go back into the lane until told to by the road signs.
If you drive in a red 'X' lane, you could get a £100 fine and 3 points on your licence.
Highways England has advice in this video on what to do when you see a lane with a red 'X'.