Isn't a RCBO a MCB and RCD combined? Which just plugs into the CSU (v.
expensive though). Is it possible to get different mA's for the RCBO?

The "best" solution would have the following

incoming SWA
|
+--- 100mA -------+--- 6A MCB --------- lights
time delay |
RCD +--- 20A/30mA RCBO -- sockets

The RCBO could be a different size (i.e. 16A/32A) depending on the size of
your main feed cable. The above is total overkill for a shed that isn't
really a workshop with dangerous machinery. The solution above is better,
because in the event of an earth fault on the sockets, the RCBO is
guaranteed to blow before the 100mA time delay unit, keeping your lights on.

For normal shed purposes, the following is cheaper:

incoming SWA
|
+--- 30mA -----+---- 6A MCB --- lights
RCD |
+---- 20A MCB -- sockets

You may be able to use the RCDs as replacements for the switch incomer on
the consumer unit. You can almost certainly buy the second solution off the
shelf as a preloaded consumer unit under the "garage" consumer unit section.
It will be much cheaper.

i.e.
http://www.screwfix.com/app/sfd/cat/...28438&id=63138

Has a 16A MCB, instead of a 20A. However, at 25 quid, it is cheaper than
just a single RCBO.

Christian.