If, say, I was breaking an old car, should I declare the vehicle scrapped when I've finished breaking it? Theoretically, what would be the advantages of not declaring it scapped?

If the scrapped one was tax exempt some unscrupulous sod *cough* might perhaps be interested in purchasing the V5 to then claim that their not tax exempt 2cv was older than it really was....

Probably not best idea to suggest that on a public forumn but buyin the v5 And doing that would be clever

Interesting point that.....is the actual cars age based on the chassis plate or the body that sits on it?
In theory, would there be nothing dodgy if the chassis of a tax exempt car, whos body had been scrapped, was put under a later, taxable, model?
As an aside, is it illegal to run a different engine number than declared on the V5 after an engine swap?

Assuming the scrapped car doesn't need to be SORNed every year, then I can certainly see no disadvantage to not declaring that you've scrapped it. I think I'm right in saying that unlike older copies, the current version of the V5 doesn't have a section to fill in if you scrap a vehicle yourself, on the grounds that vehicles should only be scrapped by licensed waste recyclers.

As to what use it could be to keep the identity 'alive', although not legal, that ID could be given to a car that for some reason didn't have one of it's own. Possible reasons for that might be, that the V5 and any record of the number plate had been lost, a car built from a collection of second hand parts, a car which was scrapped under the scrappage scheme but then sold on for parts only, a car that has been imported without any documents relating to it's age and therefore at risk of a 'Q' plate (or that the owner can't be asked to do the import paperwork), or a car that is stolen.
Although there aren't any circumstances where it's legal to transfer an ID onto a completely different vehicle, only one of the above reasons holds any moral problem for me, and that can easily be avoided by not selling the V5 to a bloke down the pub for £50.

Were someone to be theoretically doing this, they should remember to also theoretically keep the VIN plate to make things easier should the ID be theoretically needed in the theoretical future.

As to whether you should declare it when you change an engine in a vehicle, on the numerous occasions I've done so I've never bothered. You are supposed to but I've never heard of anyone facing legal consequences for not having done so.(assuming the engine's of the same type)

On a final note, while I wouldn't have any problem with 'manipulating' the paperwork for one of my own vehicles, if I was to sell one, I would tidy up the loose ends, or make the buyer aware of the history, in order to keep my conscience clear.