Sent off my new VIN (even though it isn't yet stamped on the car). It's SE7ENBEC200766SRW. Seems a quite unique mixture of letters and numbers that should hopefully suffice. As I am officially rebuilding the car, it's merely a change of chassis rather than a complete re-registration. The VIN needs to match the V5 for the MOT so that's another job done.
Now to get the number stamped onto a plate that will be secured on the car. I think it's necessary to have two separate places where it's marked, one is already going on the bulkhead. Don't really want to remove the powder coat to stamp it directly on the chassis so another little plate would ideally be hidden somewhere else.