Because the consumer unit comes under Part P only a general description is going to be given. The unit is a box which contains the fuses, Isolators, MCB, RCBO, and even in some cases relays and transformers. In the old days it held re-wire type fuses and one would normally keep a card of replacement wire. Because it was easy to use the wrong wire these were changed to cartridge fuses and physical size stopped much of the problem of wrong size fuses but caused more problems where replacements were not readily available and in turn gave way to the use of the MCB. All this did not require any change in the consumer unit itself. But next was the RCD. Either these would be mounted outside of the main unit or a new consumer unit was required to mount it inside again as we progressed to RCBO they were often too long and/or wide to fit the original unit again meaning the old box had to be changed for a bigger version.
When an up-grade is required to accommodate these new devices the house holder needs to decide which of the many options to select and it all comes down to money v conveyance.
All circuits that require RCD protection having their own RCBO is the best system and will fully comply with regulation 314 using a single RCD to protect all is likely to have nuisance tripping problems and if it does then will not comply with 314.1(iv) this is odd as the way rules are written it has to be split into circuits to reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation so if there is no possibility of it tripping it does not need splitting into circuits. Also 314.1(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit which means on can't really combine lights with sockets unless so other safety system in included so for example if you use standard lamps from power outlets as well as lights on main lighting circuit then losing either supply will not cause a danger and for example putting down stairs lights with up stairs sockets and visa versa could be accepted. It can be seen therefore no right or wrong just shades of grey and with an open plan house it may be possible by the use of one or two emergency lights or ali-tube cable to be able to comply with the regulations with a single RCD device.
However would you want to? For households who use IT equipment which in turn uses filters on the mains the more RCD's on the sockets the better. Even to the point of converting some 32 amp ring mains into twin 20 amp radial circuits in some cases. Cookers, Fridge/Freezer, Smoke alarm either are likely to trip an RCD or will cause problems or even danger should the RCD trip so either the use of Ali-tube or RCBO would seem correct way out but the Fridge/Freezer needs special consideration. If it has it's own supply although it is less likely to trip one is also less likely to notice if it has tripped and I am not convinced dedicated circuits are a good idea. Again over 10ma leakage all sorts of special earthing arrangements are required and to feed Cookers with Ali-tube may be permitted but not sure if one should or if the cooker trips the RCD one should change cooker instead? Smoke alarms have the same problem as freezer and some form of battery back-up is required to set off an alarm on power failure which in turn means it does not require it's own supply.