The number of British actors in Star Wars was purely because it was shot in London and they would have had to pay travel and accommodation for US actors, surely? It only made sense to do that for the leads.
As for Robert Shaw and Donald Pleasance, they were plenty well enough known to get roles without being hired to fulfil some weird "British quota". Unless you have a source for that?
But they used unknowns for the leads. There was no box office value to the main cast so they used Guinness and Cushing for marquee value. But--they could have used a more prominent American--they had previously--there are lots of UK-made Hollywood films through the 60s and 70s where they brought over the American star or two.
John Wayne, Carol Lynley, Gregory Peck etc... granted--the budgets were smaller but since they were spending $8 million on it, they could have done it if they felt they needed something more for marketing it.
There were many American actors who they could have used instead of Shaw and Pleasence. I don't know if Lee Marvin turned Jaws down or was too expensive or was too famous to be opposite Scheider and Dreyfuss. Sterling Hayden was another contender. Hal Holbrook could have done Halloween. There was not shortage of US alternatives.
And the UK film industry was pretty much dead by 1977 so injecting UK presence into some films--especially ones that were going to be playing in the UK, made sense in order to give it some local appeal.
Michael Caine in Dressed to Kill for example. It's not set in England is it?
How many American films did David Warner show up in as the only English presence?
I don't think it comes down to "hey, we need an English accent in this."
There must be some strategic aspect to it--because it has been cranked up ever since.
Why did they choose an English actor for Superman or a Welsh one for Batman? An Australian actor for the Joker?
That's not accidental.
They did not want an American actor for them.
PS
There is a theory that the reason like using English accents in some things is because it lends an air of authority or seriousness.
I am not sure that is always true because I don't think Quint or Loomis would be regarded with the same seriousness if the parts were played by Terry-Thomas.