Is "the bar", as in the bar that barristers work at as lawyers, capitalised or not?

I’m applying for lawyer jobs and training, so I have to use the word "bar" a lot, and I’ve noticed that small and upper case B is used. I’m afraid my English isn’t good enough to work out why. I know that "mechanical engineer" is lower-case and the job title "Mechanical Engineer" is upper-case, but for Bar vs bar I am confused.

Some examples I’ve found:

• Wikipedia refers to "the bar".
• The Bar Council refers to "the Bar" throughout its pages
• An example chambers using upper-case "Bar"
• An example law newspaper using lower case "bar"

So what are the rules for Bar vs bar?