You can do it all by post, I do quite a few a week.!!
Go to;
http://www.direct.gov.uk/en/Motoring...tes/DG_4022576
and print the form off, then send the form, correctly completed with your valid MOT certificate, signed log book and details of your tax disc - serial number and expiry date, to your local dvla office, with a cheque for £105 which covers the retention and the transfer fee onto your new vehicle, in the next 12 months. If you do not transfer it in 12 months you will have to pay another £25(i think) to update the retention certificate.
After a week or so, you will get a new registration number, a document to allow to to have the plates made up, a new tax disc, and the green slip off the V5 with the new plate stamped and manually entered on it. You will have to send your old tax disc back in the envelope providied. After 4-6 weeks you will receive your new log book with the new number on it. On an HPi check it will come up with the history of number plates and when the they were on the car.
Remember if you want to transfer the plate into someone else's name other than the owner on the logbook, you will have to enter their details as the nominee
When you want to put the plate on yor 'new' car it is a similar system, but they can do it while you wait at the DVLA office rather than waiting 7 days for the post etc - You will need to be the registered keeper of the vehicle that the plate is going onto or it has to be sent off to the DVLA in Cardiff, and takes approx 3-4 weeks!
Hope that clears this up