If you want to comply with BS7671 then it does state a protective device can be up to 3 meters from the origin of the cable if no branches, as to if this applies to unfused spurs from a ring is debated, and there is not real answer, you would have to read the book and decide your self.
However as for cable length there are two reasons to restrict the length, one is volt drop, the other is the speed that the protective device can disconnect the supply.
So with the old fuse we wanted 0.4 seconds so with a 32A fuse the impedance (AC word for resistance) had to be 1.04Ω however if you exceeded that valve then it just made it a little slower, but with a B type MCB there are two devices in one, a thermal part which takes time to heat up, and a magnetic part that works in around 0.01 seconds, so unlike the fuse, if you go slightly over the impedance which was for years 1.44Ω for a 32A MCB then it jumps to around 5 seconds so it was changed to allow a 5% volt drop so now 1.37Ω.
Now with a RCD the earth loop impedance is not so important as the RCD will trip well within the time with quite a high figure, but the line to neutral impedance should still be low enough we for some reason never quite worked out why, measure the line - neutral as prospective short circuit current so simple ohms law 168 amp.
Volt drop uses the same impedance or prospective short circuit current to work it out, but there is a grey area, the design current for the circuit Ib is normally taken as 20 amp centre and 12 amp even distributed so we work on 26 amp, but there is no rule to say we should do this, so as the circuit designer you could take another figure. Also the correction factor Ct means we don't use 18 mA/A/M but around 16.5 mA/A/M so the sum becomes rather complex, but for a ring final with a incoming loop impedance at the consumer unit of 0.35Ω we would expect the centre point of the ring final to be 0.94Ω or less which would indicate the ring final has a 106 meters of cable used.
However the only modern item which is really affected by volt drop is the refrigeration plant used in non inverter controlled fridges, freezers and air conditioners, some old radios may produce a main hum with too much volt drop, and some old fluorescent lights may fail to start, but in the main the use of switch mode power supplies has resulted in very little units that are still affected by volt drop.
So with a RCD protected supply to a dish washer, the volt drop and earth loop impedance may mean the circuit does not comply, but the danger involved both to the dish washer and to personal is minimal even if exceeded.
However here in Wales a kitchen is a special location, and work needs registering under Part P building regulations, which means some one will need to test it and say if it is within the limits set by BS7671. So although the 3 meter limit 433.2.2 and the impedance table 41.3 may not in real terms be that important with RCD protection, it is down to the person doing the testing as to if he/she feels it complies or not.
So for an electrician he will measure the loop impedance and see if it is likely to be exceeded, and then decide if heavier cable, or different route, or use of a fused connection unit is required, unfortunately the cheap plug in testers like the EZ150 show all OK at 1.5Ω so you know if really bad, but not good enough to work out how much lee way you have.
But a 13A fuse can have an impedance of 2.42Ω so using a cheap plug in tester can say if within the limit if using a FCU, so for the DIY guy that is likely the easiest way around the problem, except when using a refrigeration device, as the fuse increases the volt drop, which is why all fridge/freezer instructions say do not use an extension lead, however I am using an extension lead with not problems.
So if in England where your not registering the work, likely you will have no problem, if in Wales where some one may such air through teeth and declare jobs worth, then you need to measure first.
To sign the minor works or installation certificate you say you have the skill required and have done all the tests, in which case you would not be asking the question, so we all know 95% of DIY work will not comply, but it is still not dangerous, this is one reason the IET says should do an electrical installation condition report every 10 years or change of occupant which ever sooner, to catch the odd error, plus any degrade of materials.
So if you make that isolator a FCU then unlikely to be any problem. And when I tried to get copies of the compliance and completion certificates from council when selling last house, they could not provide them, so unless you workmanship is that bad you kill some one, in which case the council would be forced to find them, it is unlikely anyone will every worry that it is not registered with LABC.
