According to a BBC interview with LucasFilm's UK production executive Peter Beale, casting senior British actors in seeming title roles was instrumental in helping to get Fisher, Hamill and Ford work permits from the British Home Office which required letters of support from Equity, the actors union (without which the film couldn't have been made).
In short, they told the government and Equity that the film would be headed up by Prowse, Guinness and Cushing with Fisher, Hamill and Ford as supporting actors. Employing a number of British actors (along with local production staff and crew) allowed them to access cheap filming facilities at Elstree Studios and certain government tax incentives that were on offer at the time to encourage foreign companies to film in the UK.
Director George Lucas had cast a trio of young unknown American actors
in the lead roles of his space adventure and was gearing up to shoot
in Britain. But his UK production executive Peter Beale knew Britain
probably wouldn't let them.
"Equity was trying to look after the British actors; there was no work
around and they didn't want foreign actors coming in and taking
midsize roles or big roles that the British actors could do," he said.
But Harrison Ford, Carrie Fisher and Mark Hamill needed work permits.
So Beale, like others involved in the blockbuster's production, had to
get creative.
He drew up an alternative cast list showing the film's British actors,
including Alec Guinness, Peter Cushing and David Prowse at the top,
and Ford, Fisher and Hamill in apparently minor roles.
"I went to Equity and said 'Look, the English have got the best parts
in this, but the director wants three little Americans for the smaller
parts' and I also went to the Home Office and told them the truth...
and they supported it because they wanted the work and they recognised
it."
What did Equity say? "By the time they realised, I think they had
forgiven me."
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Note that Lucas was keen not to cast British actors if at all possible so the pressure on him (to do so) was largely external rather than a desire to meet stereotypes about British actors playing villains well.
Whenever there was a break in the recording, Lucas would run to London
to loop Alec Guinness, Mark Hamill, et al.—though at least a couple of
actors read their lines at Anvil. Anthony Daniels arrived for
voice-over work despite the fact that dozens of others had auditioned
for the speaking role of C-3PO. “It was primarily because of the fact
that it was a British voice,” Lucas says. “I really wanted to keep the
whole thing American. Tony had the most British accent, so I said,
‘No, I want to make him American because he is one of the lead
characters.’ I wanted Threepio’s voice to be slightly more
used-car-dealer-ish, a little more oily. More of a con man, which is
the way it was written, and not really a fussy British robot butler.
So I tried and tried, but because Tony was Threepio inside, he really
got into the role. We went through thirty people that I actually
tested, but none of the voices were as good as Tony’s, so we kept
him.”
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