How to extend wiring for extra high current sockets/appliances in Utility room
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.