Its the installation methods and cable length that you use that determins the MCB size , you are incorrect when you say the on site guide shows 12 amp , table 4D5 BS 7671 , gives current carrying capacities and instalation methods ,and clipped direct a 1.5 mm cable will carry upto 20 amps , how ever you should never protect a 1.5 mm cable with that size breaker , voltage drop you are allowed 3% diversity so on long cable runs the voltage drop can be quite considerable , the other thing you look out for is the grouping factor when all the cables are grouped together ie on a cable tray in conduit , trunking etc , but the real crucial thing is insulation ,as i have mentioned clipped direct a 1.5mm cable can take 20 amps , stick it in trunking even a small piece then the current carrying capacity drops to 16.5 amps or cover it it no more than 100mm of insulation with one side touching plaster board then it drops again to 16 amps,enclose it in conduit in an insulated wall and it drops again to 14.5 amps ,cover it with more than 100 of insulation with one side touching plaster board and its then rated at 13 amps ,but if its surrounded by insulation even on a very short peice of the cable run then the cable rating will drop to 10 amps , hence the reason you tend to use 6 amp or 10 amp breakers and no higher , so in general terms its the maximum loading , cable drop ,installation methods , you can put socket outlets on 1.5mm as long as they are protected by a fused spur and as long as the installation methods are not 103 !one of the other items is the Zs ,although quite high 7.66 on a type B 60898 breaker the higher the current and type the lower the Zs readings so these should also be taken into account .1 hope that explains your question check out table 4D5 of the regs all revealed there
