Electrical – How practical is it is use a 6 Terminals junction box to extend a ring main

TL;DR -- find the breaker that shuts that red wire off, then slap the builder with it, so to speak

The builder of your house needs a slap with the NEC. There are two very clear Code violations here, and they're both things that are trivial to avoid.

First off, the lack of panel labeling isn't just a massive inconvenience to you, it's against Code -- the NEC requires it so that the next electrician/homeowner who looks at the panel can actually figure out what breaker to shut off to make a circuit safe or get that sparking oven to quit. In particular, it'd get dinged for busting 110.22(A):

110.22 Identification of Disconnecting Means.

(A) General. Each disconnecting means shall be legibly marked to indicate its purpose unless located and arranged so the purpose is evident. The marking shall be of sufficient durability to withstand the environment involved.

and 408.4(A)

408.4 Field Identification Required.

(A) Circuit Directory or Circuit Identification. Every circuit and circuit modification shall be legibly identified as to its clear, evident, and specific purpose or use. The identification shall include an approved degree of detail that allows each circuit to be distinguished from all others. Spare positions that contain unused overcurrent devices or switches shall be described accordingly. The identification shall be included in a circuit directory that is located on the face or inside of the panel door in the case of a panelboard and at each switch or circuit breaker in a switchboard or switchgear. No circuit shall be described in a manner that depends on transient conditions of occupancy.

Second, what you have is indeed a Multi-Wire Branch Circuit (MWBC) -- the 52V on the black wire with breaker #23 OFF is stray capacitive coupling from the still-live red wire. The red wire, of course, is still live because of another Code violation, this time of 210.4(B):

(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

So, it's time to find the breaker that turns off that darn red wire (leave #23 off while you do this!). Hopefully it will be adjacent to Breaker #23 -- in that case, you can get a handle-tie from the electrical supply house and handle-tie the two dodgy breakers together to fix the issue as per 240.15(B)(1). If the builder was as clueless as I fear though, that breaker is somewhere totally else in the panel -- something you'll want an electrician to fix.

Qu 1: What is the best way to extend the existing double socket to add 3 extra sockets.

You want to increase the total load from 15A to >27A.

You probably don't want the breaker tripping if you, for example, run the washer and dryer at the same time.

If the existing circuit is really a radial circuit with a dedicated breaker in the CU you will need to check

• The breaker rating
• The wire diameter.

If the wire diameter is small, you cannot increase the breaker rating without first replacing the wiring with a suitably sized new circuit. Under UK regs I believe you should get an electrician to do this work.

I can't use a spur from existing socket as this will pull too much current.

It depends what you plug in. The breaker should protect the circuit.

It sounds better to continue this as a ring main rather than a radial circuit.

Ring mains are typically 32A but I don't see creating a ring main as easier than upgrading a radial circuit.

Using a double socket for washer/dryer isn't great as they don't deal well with 20A+.

So long as the circuit is properly constructed, you should be fine. I certainly run those two appliances from a double socket.

Qu 2: Can I just connect the 3 x 13A switches in parallel from the existing wires coming from the consumer unit?

You should draw a diagram of your intended circuit. You don't wire switches in parallel. I guess you mean something different but you need to be clear.

You may be over-complicating things here. You can use normal switched sockets (even though inaccessible with appliances in place) and when necessary, just isolate the whole circuit using the switch on the breaker.

Qu 3: I presume if I want a normal double plug/switch (above the worktop), I can add this in parallel as well.

Yes.

Qu 4: Presume makes these changes will need me to get an electrician to certify it (even if I do the work myself)

As I understand part P, You can extend an existing circuit but you need an electrician to certify it if you create a new circuit. It might be no cheaper than paying an electrician to do the work themselves.