I think that cable is most likely 10mm² (which is fine, it's not a problem to have a cable larger than it needs to be as long as it's not so large that it can't be reasonably terminated).
In typical modern UK T&E conductors up to 2.5mm² are solid while larger conductors (4mm² and up) are stranded. In typical modern 4mm² and larger T&E the earth conductor is two sizes smaller than the current carrying conductors.
Therefore
1mm² and 1.5mm² and 2.5mm² T&E has all conductors solid.
4mm² and 6mm² T&E has stranded current carrying conductors but a solid earth conductor
10mm² and 16mm² T&E has all conductors stranded.
Note that this applies to modern T&E. Your cable has harmonized colors and therefore must be modern.
What concerns me about your photo is I see no evidence of a backbox, on the other hand I see no evidence of any screwholes either. Was there a backbox that you removed for the photo? or was the socket somehow installed with no backbox at all?
Sorry to confuse you all. Yes I am guilty of removing the metal back box. . Great answers throughout thread though and Ihave learnt lots. Thanks to all
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