Perhaps Bentley should have some sort of connector block under the car or concealed in a wheel arch for connection of a remote power source or battery booster. It can't be a rare occurrence.
On the GT's, the right hand rear seat base and back come out and there is manual boot release behind the seat . . . usually very stiff though due to lack of lubrication! Don't know if the FS is the same.
Also try holding the key in the start position for 15 seconds.
Had the same problem on my Mum`s Flying Spur last week. Just wanted to say thanks to Phantom Motors for coming out and fitting a new battery so quickly.