OK, I'm now none the wiser on the 'x2 radials' idea but assume it's a no-go.
So, back to my original scenario: a new ring circuit, 2.5mm t&e, c. 22m end to end, not running through any insulation, feeding only a 1.5kw water heater and a (let's say) 2kw underfloor heating system. Because the...