You are allowed to do anything you want under Part P if you pay the council their fee and are skilled enough to do it.
We don't know your skill so can't say if you personally can do the job but the real question is "Is it worth it?" Fees vary but let me assume £70 plus you will need to hire test equipment at another £70 by time you add the £140 to the job it is often cheaper to get a registered electrician to do the work. You may be able to get council to do the testing for you again depends on council.
Rules say you should test and submit results if they don't like them then they can re-test at their cost to verify in some cases they will just test but depends on council.
There is a link on the projects section to Part P
As to what if you don't well insurance may not be valid and you may have problems if you come to sell the house.
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