It was not obvious from your original post that it is already supplying a CU!
As a sub-main in SWA you have more choices than if it is a final circuit. Specifically you don't need RCD protection on SWA (as the armour protects against cable penetrating damage) and as a sub-main you are allowed up to 5s disconnections times (so you can use a fused-switch to feed it and achieve selectivity with the downstream MCB/RCBO).
Basically having selectivity means that a fault on a final circuit will only trip the local CU's MCB/RCBO, and not take out the whole lot. That is a special risk if the shed is used as a workshop and being plunged in to darkness a safety hazard!
Once you put a socket on the 6mm sub-main it must have both 30mA 'instant' RCD protection and 0.4s fault current disconnection times. At that point you won't be able to achieve any useful selectivity on either earth leak faults or on over-current faults.
Now that may well be how it is currently wired, and if you are OK with that then fine. But if adding circuits to the CU in the future, which I think is notifiable work, you may find most electricians are unhappy at the inability to design/modify for adequate selectivity (as required by the wiring regulations) without having to undo your work.
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Basically you would be well advised to follow telectrix's advice in post #6 and simply run another feed to an outside socket from the internal socket circuit.
If you want proper selectivity it would be notifiable to change the overall CU configuration, but simply running your new socket feed to the existing MCB would make that job much easier in the future. Then any electrician would have the cable there to put it on a separate circuit, RCBO for all if needed, and change the sub-main feed to a fused-switch.
But again that is for future consideration.
