Yes you can add a new socket to an existing circuit yourself.
Answered by Rupert, Electrical Safety Expert
You are allowed to add a new socket to an existing circuit yourself, but if you do not have the knowledge and skills to do so and need to look online for a tutorial, then you should not be carrying out the work.
We recommend that you use a registered electrician for all installation work. This provides the assurance that it has been done to the correct standard (BS 7671) and that in the unlikely event that you are not happy with the work that you have a route to getting the problem sorted through the complaints system operated by electrical scheme with whom the electrician is registered.