Yes fine in my view, to require that 5 amp junction boxes be replaced when a 5 amp fuse is replaced with a 6 amp MCB is IMHO over pedantic.

The load probably wont exceed 5 amps, and with the general use of low energy lamps is unlikely to exceed even half that.

And in most situations the junction box wont be carrying the entire load of the circuit in any case.

And even if a full 6 amps did pass, I doubt that the terminals would even get warm. Most of the current will pass from one wire to the other wire when these are forced into close contact by the screws. Very little current will pass through the metal parts of the terminals. 

I would take the “5 amp” rating as meaning “not big enough to take the size cables used on power circuits” Rather than meaning “liable to overheat at over 5 amps”

Yes fine in my view, to require that 5 amp junction boxes be replaced when a 5 amp fuse is replaced with a 6 amp MCB is IMHO over pedantic.

The load probably wont exceed 5 amps, and with the general use of low energy lamps is unlikely to exceed even half that.

And in most situations the junction box wont be carrying the entire load of the circuit in any case.

And even if a full 6 amps did pass, I doubt that the terminals would even get warm. Most of the current will pass from one wire to the other wire when these are forced into close contact by the screws. Very little current will pass through the metal parts of the terminals. 

I would take the “5 amp” rating as meaning “not big enough to take the size cables used on power circuits” Rather than meaning “liable to overheat at over 5 amps”