To add to the-knobb's excellent answer to Heathfield. it should be noted that the BS7671:2008 17 th Edition wiring regulations are NOT enshrined in statutory law - so not complying with them is not "illegal". They are guidelines for best practice, and it is suggested that complying with them is the only way to fully to meet Part P of the building regulations, but they are NOT "law".
"The technical requirements laid down by Part P are remarkably simple. They are:
P1 Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury".
"Surprisingly, perhaps, Part P does not alter the status of the BS7671:2008 Wiring Regulations. They are still non-statutory, and there is still no legal requirement to adhere to them in domestic installations. This is a source of great discomfort to many electricians, and some of them will jump through hoops to try and prove that they are mandatory, but the fact remains that they are not.
HOWEVER, adherence to them is a very good way of ensuring that you meet the technical requirements of Part P, and you would need a very good reason, and a very good understanding of what you were doing, to decide to not adhere to them. "
The only electrical "law" is the Electricity at Work Act, which applies to me, as a tradesman, not you as a DIYer.