Method. Size is the smallest conductor where the tabulated rating It ≥ In ÷ (Ca × Cg × Ci × Cf), voltage drop is within the limit, and Zs ≤ 0.95 × 230 ÷ Ia for the MCB. Ib is checked against In separately.
Voltage drop uses the full tabulated mV/A/m (impedance z from 25 mm²). That is the conservative route; Appendix 4 lets you reduce it when the cable runs cooler than its maximum.
Zs uses copper resistance at 20 °C from GN3 Table I1, × 1.20 for 70 °C cables and × 1.28 for 90 °C. Armour resistances are typical figures; check the maker's datasheet for the cable you fit. Fuses and RCD-only (TT) protection are not checked here.
CPC adiabatic check uses S = √(I²t) ÷ k at the largest earth fault current, which is at the board (1.1 × 230 ÷ Ze, or your measured PEFC). I²t is the smaller of I² × 0.1 s and the BS EN 60898 class 3 limit at that current; below 3 kA the 3 kA limit is used. k is 115 for the T&E CPC, 143 for an XLPE core and 46 for XLPE SWA armour. Armour areas are typical; check the datasheet.
Not covered: harmonics, aluminium, single-core cables, and rings. Check final designs against your own copy of BS 7671.