So what about Dave Prowse, the actor who actually portrayed Vader on the big screen? According to the actor's Twitter account, it's not him is right. Jones in Vader costume-He NEVER did press for #SW & certainly never in #DarthDrag! WRONG! Never heard of so-called'ban'-but that's NOT Mr. Mark Hamill, the actor who played Luke Skywalker, and who appears in the photograph, tweeted:

As such, it would have been unusual for Jones to wear the costume for a promotional event. Getting back to the photograph in question, however, the man in the Darth Vader suit is not James Earl Jones. Although Jones is well-known as the voice of Darth Vader, he didn't actually physically portray the dark lord on the big screen. The 1965 Race Relations Act prohibited 'discrimination on racial grounds in places of public resort.' For starters, there were no 'strict segregation laws' in London in 1979 - there were no segregation laws in England at all. This claim appears to have been made up out of the whole cloth. A photograph purportedly showing actor James Earl Jones dressed as Darth Vader on the streets of London alongside other Star Wars actors (sans costumes), is frequently shared with the strange claim that Jones had to socialize incognito due to London's 'strict segregation laws':

Darth vader actor james earl jones