I am not a professional electrician, so what follows is based on my own understanding of the situation.
A standard ring circuit (ring main) consists of a loop of cable whose live and neutral cores are of 2.5 square mm cross-sectional area (csa). The two ends are connected to the same MCB in the consumer unit. This MCB is often a B20. This will sooner or later trip out if there is an overload (how soon depends on how much overload). In the case of a simple overload, tripping out is performed by the heating up of a bimetal strip inside the MCB. The greater the overload the more quickly it gets hot, so the sooner the switch trips off. If there is a massive overload - a dead short - there is no delay. A second device in the MCB operates - typically in the case of a dead short. This is an electromagnetic relay,and trips the switch instantly.
B-rated MCBs trip quite quickly on overload. C-rated ones trip more slowly. If you have a circuit for an appliance with a powerful motor which draws a very high starting current (eg air conditioner, workshop air compressor) you may be told to fit a D-rated MCB.
For some reason which I do not understand, C-rated MCBs are used in the small mains consumer units fitted to touring caravans. But there is another aspect of the MCBs used for these installations which is completely understandable, and which I strongly believe should characterize those used in domestic installations. Caravan MCBs are two-pole. When the bimetal or electromagnetic device operates to trip off the MCB's switch it operates on the live (or "line" or "phase") side of the MCB, as in a domestic MCB. However, it switches off the neutral as well. This is not so, or not yet, as far as I know, on domestic consumer units. True, there should be no human safely issue on a circuit if the neutral is not disconnected when the MCB trips or is manually switched off, as neutral and earth should have only a very tiny potential (voltage) difference between them. However, if, as is the case on all but very old installations, at least the "power" circuits are protected by an RCD (residual current device), you cannot easily work on a circuit with just its MCB switched off. You often end up doing what you should have done in the first place: tripping off the RCD, and thereby switching off all the "power" circuits! This is because, when changing a plug, say, it is very difficult to avoid letting the earth and neutral cores of the cable touch each other. The RCD trips if there is difference of more than 0.030 Amps (30 milliAmps, or 30 mA) in the current flowing in the live and neutral cores. The live is switched off, so the RCD "expects" to see no current flowing in the neutral core. Touching this core to earth makes a tiny current flow in it, because of the small difference in voltage between neutral and live (0 Volts) and neutral and earth (probably just a few milliVolts). If this current exceeds 30 milliAmps the RCD switches off.
A ring circuit with 2.5 mm cores is equivalent to a radial circuit with cores of twice the csa. You don't find radial circuits for sockets in modern installations - only for lights, and for electric cookers. For sockets, the ring circuit is much more versatile than a radial of the same capacity as you get twice as much cable length for the same total load. If you like, it's like a 5 mm cable split into two and taken by two different routes from the same fuse up to where the two ends meet.
Your question seems to relate to a section of one of your ring circuits which supplies your kitchen. Your electrician has probably provided a spur from a point on this cable to supply an extra socket, or a fixed fused connection unit (FCU - often called a spur box). The latter might be to supply a cooker hood, or something else which is not portable so does not need a plug and socket connection.
If this is the situation, I see no problem with the junction box used by your electrician. He would be very unwise to use a component which is not suitable for the load. The load on the spur cannot possibly be expected to be as high as 20 Amps. The boxes which I have used for such purposes (in the happy days when I was allowed to do my own installation work!) are indeed rated at 30 Amps, but this is probably because the trade counter where I bought them sold only this type for "power" circuits.
Only if you need a circuit for an electric cooker do you need to think in quite different terms. These are supplied from a 32 Amps MCB (also B rated) and use a single radial cable of 6 square mm csa. There are no ready-made junction boxes for this size of cable, as far as I know. I you have to extend such a cable, use a box in the wall (or a surface mounted pattress) covered with a blanking plate. Join the cable inside the box/pattress using a suitably large capacity strip connector.
The above covers rather more than you need, but is hopefully helpful and potentially (no pun intended!) useful.
