Electrical Malfunction In My Kitchen, Please Help?

It sounds as though your boiler switch is in fact a "spur"; that is to say it is fed from one of the sockets you changed. You can tell which sockets are connected to a spur - they have THREE sets of cables connected instead of the two you find on a normal ring main. I suggest that you have failed to reconnect all three cables (perhaps left a wire dangling). Switch everything off and check each socket in turn until you find the culprit. It's likely to be the socket nearest to the boiler cupboard. The fuse for the boiler should be 3 amp but it only matters if there is a fault in the boiler - a bigger fuse could cause the wiring to overheat under fault conditions before it blows