Is it OK to put standard "twin & earth" cable directly into the cavity of a dwarf wall?

The good news is that the insulation isn't in yet, so I can just leave the insulation out. Its debabtable that I was ever going to put insulation in, a conservatory by definition, isn't an efficient structure anyway and more heat is going to be lost through the glass than the brick work.
But I am surprised that a 2.5mm cable in this secenario isn't suitable for a ring main.. I am going to dig out the regs books and see what you are talking about, as surely its common for a 'ring' to be buried under insulation at some point around the house.
I can't run it back to the distribution panel, as its too far away and I can't downgrade the MCB because its the ring for my whole downstairs.
But surely 2.5mm could carry more than 32Amps when in a ring even if under insulation.. Sure, if it were a radial circuit (therefore only one cable) then a 2.5mm cable won't carry the current. But as its a ring the power will be 'shared' along 2 different cables.. Surely a 2.5mm cable can carry 16amps, under any circumstance?
With regards to having cables on the outside of the cavity goes (i.e the inside of the conservatory), its a complete no go.. I have spent a lot of time and effort getting all of the cables and back-boxes in during the build and I am not going to change this now.
I appreciate your help, but I need to know more now, as based on what you have said, I have done it all wrong!!!
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