FR always divert to a base where possible as you don't have to make ad hoc arrangements with handlers and engineers. You also have crews on standby to reposition the aircraft when you can. FR don't use CWL if they can use BHX or BOH for that reason and nothing to do with history.
CWL is not listed as a div on FR flight plans but I know the BRS guys always keep CWL in the back pocket as a bolt hole as BHX has a pretty poor fog record and BOH base is shut in fog season so there is a good chance you might get the odd BRS divert especially with an easterly wind !
It's not political it's operational.