It's probably been re-registered after a period off the road. If you can provide evidence of the original registration (e.g. log-book) the original number can be re-issued.
I'm guessing the car was rebuilt from a very bad state and the original registration lapsed.
You can reclaim a registration for a vehicle if you can prove its engine and/or chassis match, relatively easy for a Bentley where records are likely to be good.