Hey Bobnick
I don't think you need worry about overloading the Main Switch, it can happen but not often, your service fuse should be in-line to protect against that sort of overload.
Stick with the board, arrange it so the loads are in ascending order, e.g. start with the biggest current's closest to the main switch, and the same from the RCD side, this is good practice.
I would reccomend that you arrange the MCB's in this order
MAIN SWITCH 100A
Cooker 40A
Immersion 16A
Central Heating 16A
Ground Floor Lights 6A
Kitchen and Utility Lights 6A
First Floor Lights (1) 6A
First Floor Lights (2) 6A
RCD - 100A - 30mA Residual current
Garage 32A
Ground Floor Power 32A
Conservatory Power 32A
Kitchen and Utility Power 32A
First Floor Power 32A
External lights 6A
Now theres a few thing's you need to know, firstly all the breakers should be B-Type and not C-Type.
Only the ground sockets need be on the RCD, all the rest are up to you, as a rule of thumb I put all socket's on an RCD, it will only trip if you have a fault.
If the garage has an RCD of it's own in the consumer unit (CU) in there (Garages usualy have a small CU of there own) then there is no need to put the garage on the RCD side of the board in the house.
If however there is no RCD in the garage then leave as is above.
You need to look at Main Earthing and size of tails, for a 100A supply you need to be looking at using 25mm sq double insulated meter tails.
For the Main earthing to the Gas and Water Pipes and any other services you will need 10mm sq single core copper earth cable, this links from the pipe, e.g. water, back to the Main Earthing Terminal (MET) which should be located near the consumer unit.
Supplementary bonding, this is additional earthing to the main bonding, this is usualy carried out in the bathroom, it links all the pipes to each other and then to an earth teminal such as in the light switch or light fitting, this is not run back to the MET, and is run in 4mm sq earthing cable, all of the bonding should be donw with appropriate earth bonding clamps to BS 951.
A few other things to consider
Have you made sure all the cables are the correct size for there application?
e.g.. 1mm/1.5mm for light circuits, 2.5mm for a 32A Ring circuit or 20A radial and so on?
If theres any other questions you may have please feel free to ask
Earth Block -
Earth Clamp -