I am looking to replace my consumer unit with a split load Wylex 12
way Consumer unit, since i have run out of ways on my exisiting
consumer unit. At the same time I want to run a spur to a shed for a
small ring main and lights. The shed will have my old consumer unit
(Which has a 63A/30mA Incomer providing RCD and Isolator
functionality)

Is it acceptable/possible to connect the spur to the shed via an RCBO,
or a second dual pole isolator or should I just use a standard MCB?
Also what rating would be suitable for running a spur to a shed, would
a 16A MCB/RCBO be suitable? (Internaly I was going to use 2.5mm t&e
ring main cable, while externaly I was going to use 2.5mm armoured
t&e)
My worries are that if there are problems on the external wiring for
any reason, then there are safety issues by just connecting it via a
standard MCB?

As a slightly seperate point does anyone know if it possible to add a
second RCD to a wylex unit, so as to provide aditional seperate RCD
protection for the lighting circuits from the ring mains?