The regulation on this is:-
433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor. or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:
(i) It is protected against fault current in accordance with the requirements stated in Section 434
(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also Regulation 434.2.1).
2.5 mm cable is normally good for about 20A and normally a ring main is protected by a 32A MCB. So where a spur is taken that spur is protected by the 13A fuse in the plug or FCU which it is terminated with rather than the 32A MCB. It is therefore important that the cable is unlikely to get damaged as shown in (ii) above. Also very limited as to length not over 3 m.
Although a spur under BS7671:2008 can be taken from the consumer unit or a socket there is an issue with Part P when doing work on a consumer unit and possibly you will either need to pay the LABC their fee normally £100+ or employ a scheme registered electrician.
Of course people do break the rules but since you are asking I will assume you intend to do the job right. This also raises the question of RCD protection for buried cables and the testing of the work once completed. The latter I see as a problem with nearly all DIY work as the test gear costs around £750 and even to hire is around £75 for minimum hire time of a week. In real terms the provision for people to DIY electrical work allows electricians to do work not covered by their employers registration. i.e. working in their own house. But for the normal person to comply it would be very hard.
That is not to say that there is anything wrong with DIY work. But we just don't know if it is wrong or right. Owners will always try to reduce costs but in rented accommodation to do anything to the electrics leaves to occupant libel if not done to the letter of law and regulations with no insurance. This of course could also be case with owner.
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