Provided the vehicle that you are scrapping still has a valid MOT and is currently licensed you may transfer/retain the registration mark.
If the licence disc on the donor vehicle has expired you may still be able to apply to transfer/retain its number without renewing the licence, provided, the vehicle licence expired no more than 12 months prior to the date of the application and a valid SORN declaration has been made. You should be aware in these cases it is likely that the DVLA local office where you make your application will wish to inspect the vehicle.
The plate is worth whatever anyone is willing to pay for it. I really have my doubts as to whether anyone would have any interest in it.
Not unless they consider fiddling with the fixing points of the plates and being imaginative with a slightly different layout of the letters.
i.e... using colored caps on the fixing screws means you might manage to turn the
.. 7 UNT.. into something resembling .. ? .. Will leave it to your imagination.
(But we are thinking of changing the look of the diagonal bar of the 7)
There's only one kind of person who would want a plate like that !