Well, the question of whether or not you need permits, and the exact permit you need, depends on the nature of your project. Across the country, rewiring projects typically do need some form of approval under the building regulations.
Personally, I believe the best thing you can do when it comes to a wiring project in London is to work with the best electrical contractor you can find. Choosing competent contactors we will take away all of these burdens from you because such professionals are more equipped to handle the permit requirements.
For instance, when I worked with Electrical Contractors Surrey, they were the ones in charge of notifying the local building control department and confirming that everything complies with electrical safety standards. Failure to do so may result in penalties.
In short, ensure that you work with a competent contractor, and you won’t have to worry about all of this.