For a 2.5mm2 radial circuit supplying a washing machine (via a double 13A socket) and a boiler/pump/valves (via 3A FCU) would a 16A or 20A MCB be 'best', and why?

If the spare 13A socket might end up being used for something, e.g. an occasional iron, does that sway the decision towards the 20A MCB?