There are two sets of regulations, the law is called Part P, but most of the law is based on BS7671 a joint British Standards and IET regulation which is not law, but can be used in a court of law.
Where the waters get really muddy is where the property is to be rented, there are now laws about renting properties which may require more stringent inspecting and testing than with the same property with an owner occupier.
As yet it seems where there is a mortgage it is still considered as an owner occupier even though the occupier is really renting the property from the mortgage lender.
So in real terms owner occupiers can still do what they want, even if they break the law, there is in real terms little chance of getting caught so may as well not exist.
What is therefore more important is BS7671 which unlike Part P is not designed to replace the Union closed shop agreement, but is there for the safety of people using the premises. So looking at BS7671 that requires any premises to be inspected and tested either as a continuous process or with a set time, with domestic that time is every 10 years or on change of occupant which ever is shorter.
So if following BS7671 the inspection and testing should have been done when you bought the flat. This would one hopes highlight any problems, the property does not need to comply with the latest regulations (BS7671:2008 amendment 3) but it is normal to mention where it will no longer comply as this will likely impact on any additions and alterations. For example no RCD protection could mean adding an extra socket will cost 10 times what it would if already upgraded to that standard.
So if there is a periodic inspection report already done, then changing a consumer unit the person changing it would test what he has done, plus a quick test of existing circuits to show the existing EICR was valid. He may want to investigate further should the results shown in the EICR not line up with his tests.
However if there is no EICR then the installer of the consumer unit would want to test the whole house. He will have become responsible for everything after the consumer unit. In real terms normally he actually does the EICR first before he changes the consumer unit, as some faults could be corrected by the method he uses to wire up the new unit. Also it allows him to raise the question of cost to upgrade circuits with the person ordering the work before it is started, rather than argue about it after.
Now we also get to where the BS7671 becomes law, most domestic electricians are members of a scheme, this allows them to issue certificates without going through the LABC, however as part of becoming a member of the scheme they agree to follow BS7671 so for them the BS7671 is law.
The distribution box today is type tested and is called a consumer unit, it has been upgraded many times, and from the 1950's we have moved in the main from fuses to MCB's the latter needs more careful testing, then we added ELCB's at first to cope with poor earthing systems and latter for more protection from electric shock, so to start with they were voltage devices, these got banned and replaced with current leakage devices, and these had the tripping current reduced from around 100 mA to 30 mA and then we also started to combine the RCD (new name for ELCB) with the MCB and call them RCBO's.
The house I am in now has 2 RCD's and 4 RCBO's with two consumer units, one dedicated to kitchen. The big thing when changing a consumer unit is the balance between the chance of tripping to cost of the unit. All RCBO's is likely the best, and most expensive, two RCD's is likely the cheapest but you are more likely to get it tripping. Deciding what system suits is likely the hardest thing.
Even as an electrician I am not 100% sure what is best, when lights don't work you know there is a fault and you correct it, when some sockets only don't work, it can take a long time before you realise there is a fault, by which time freezer food has defrosted. So although with RCBO's it is less likely to trip, it is also less likely you notice when it does trip.
So question one is why do you want a new consumer unit? Will it really help? With a child of 14 who was taking up Radio as a hobby and passed his RAE to become a radio ham having my house with all RCD protection what really some thing I needed to do. However my father-in-law with no children still today has no RCD protection. And he does not play with DIY so he is less likely to get a shock in his house than I am in mine.
My mother who put an extension lead in a bucket of water because she thought it was on fire when she saw the red neon clearly showed she needed RCD protection fitted. But every family is different, we have lived in houses for over 100 years without RCD protection as so you really do need to assess the risk and decide if you need RCD protection or not.
