Bathroom? who said anything about a bathroom?
Bathrooms, Sockets under 20A and not labelled for specific use, and cables non in exempt list and buried under 50mm need RCD at 30ma protection.
How does one say this without using the word "Bathroom"?
There is nothing in 17th Edition which says exactly what work would mean the system needs updating however it does indicate that with pure inspection and testing one would just give it code 4.
I would consider any work which would be covered by a Minor Works or Installation Certificate will need to comply with 17th Edition. Anything which would be out of the extent of installation covered by the certificate or listed in details of departures, if any, from BS7671:2008 could be to old regulations.
So looking at original question the socket and cable moved would need to comply. So there are three methods.
1) Use a RCD/FCU at original point.
2) Use surface cable and RCD socket.
3) Use permitted cable (i.e. Ali-tube) buried and RCD socket.
The problem is Part P and BS7671 have different meanings to what is a new circuit.
With Part P it is a new circuit if it is new supply from consumer unit.
With BS7671 it is a new circuit if it has it's own protective device so using a FCU becomes a new circuit.
The Installation Certificate allows one to easily define the extent of installation but the Minor Works Certificate which is suppose to be an easier short form does not allow one to define the extent but uses the word "Works" which I would consider means it only covers what has been worked on.
For the DIY person "Part 3 : Essential tests" becomes a problem as Earth continuity, Insulation resistance, Earth fault loop impedance and RCD operation need expensive meters to test and likely that the only test they can carry out is the polarity test as there are some cheap plug in testers which can do that. So in the main the Minor Works Cert will be blank but I suppose they have do it to "best of my/our knowledge and belief"!