It’s really not that big of an issue. These planes use commercial airports all the time.
as for the size of the aircraft Japan will also come by 777-300. Germany could use their A350. Korea has been invited so they would likely use their 747 too. Biden isn’t the only one with a big jet.
There is even a large chance they will go to London and then just use cars/coaches to go down to Cornwall. This is what has been done for State Visits by some leaders that have gone outside London.