15mm tube will carry a 6kw load @ 10c dt. (0.14 x 4.2 x 10c = 5.8 kW)
But the OP is only fitting 2.6kw worth of radiators. Half of what you suggest is OK.
So 15mm will carry the OP's load fine and dandy then?
Or is it actually oversized if 20dt is used?
Mebbe some cheap 10mm microbore would do at 20 dt?