What is this please, and can it be used to add an electrical socket in some way?

[EFLImpudence]

1mm² T&E has a maximum CCC of 16A.

 

[Jurassicspark]

If it were a spur off the ring, that s/would be a 2.5mm cable,

That would be ideal standards, wouldn't it?

it would have a rating between 20 and 27amps.

Again. Ideal standards.

The boiler would be consuming a couple of amps, the maximum for the 13amp socket, a total load of 15amps, as a worst case scenario.

What if the boiler's plug is removed from the socket and someone comes along and pops in his high consuming wattage devices?

More than 15A.

It's unreasonable to assume any such thing, but is always good practice to always check. Perhaps the OP could take the FCU off, and report back?

Indeed. Or by simply looking at the specification on the outer cable.

 

[Ragnar_AT]

I don't think the flex between the FCU and the boiler gives any evidence on the size of the cable feeding the FCU. Since the flex will be protected by the fuse in the plug, the only concern is whether the cable inside the wall is sufficient to supply a double socket with the boiler and a hypothetical 13 A load plugged into it and whether it is appropriately protected against overload without the fuse in the FCU.
Realistically you need to check the size of the wire feeding the FCU and the rating of the MCB it's fed from.

I wouldn't assume that the FCU is controlled by a thermostat, assuming this is the power supply to the boiler. All boilers I've seen have a separate control circuit for the thermostat and a permanent live feed to the boiler.

 

[Harry Bloomfield]

13 A load plugged into it and whether it is appropriately protected against overload without the fuse in the FCU.

If it's the boiler, plugged into the 13a twin socket, the boiler could just as easily be protected by the fuse in the plug, or the fuse in the existing FCU - there is absolutely no difference.

 

[Ragnar_AT]

If it's the boiler, plugged into the 13a twin socket, the boiler could just as easily be protected by the fuse in the plug, or the fuse in the existing FCU - there is absolutely no difference.

As far as the boiler and its flex are concerned that’s exactly what I wrote.

However, in theory the fuse in the FCU could also serve as protection for an undersized spur from a ring final circuit to that FCU or some other bodgery. Only in that case replacing the FCU with a twin socket could create any new dangers. So we need to know the size of the cable feeding the FCU and of the MCB or fuse protecting it.