As far as i am aware you can do it instantly as long as you have the correct documentation with you.
If your not putting it straight onto another vehicle you have to put the plate "On Retention" at a cost of approx £80 ish
This can be done at any Vehicle Registration Office, the nearest being Nun's lane in chester.
How can I retain my registration number?
You will need to complete a V778/1 - Retention of a Vehicle Registration Number application form. Application forms can be obtained from DVLA’s network of local offices. The cost to retain a registration number is £105.
How do I transfer my registration mark?
To transfer a registration mark from one vehicle to another you will need to complete a V317 - Transfering a Vehicle Registration Number application form. Application forms can be obtained from DVLA’s network of local offices. The cost of transfer is £80.
Find full information at DVLA Web-site
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