I cannot see any reason for putting them in a specific order - these days.
AIUI, with some of the older fuse boards, some of them had a single high current space next to the main switch which could take a higher rated fuse than the rest of the ways. So I guess this created a "rule" that the high power loads go next to the main switch, and this "rule" had been passed down from tradesmen to apprentices ever since - with no-one really thinking about the "why ?"
There is an argument for spacing "heavily loaded" devices. MCBs or RCBOs that are loaded will heat up, and the heat will affect (reduce) the tripping current of adjacent devices. Putting a number of such loaded devices all together (very slightly) increases the risk of nuisance tripping.
I'll add that SBS Trade Sales charge ÂŁ16 net for compact RCBOs (single module, 2 pole switching) - type AC or A, curve B or C.
There's a difference between "recommended" and "most cost effective way to meet regs". In practical terms, for most installations you need 2 RCDs minimum to meet specs - and back when dual-RCD boards came in, RCBOs were either non-existence (for some makes/models) or just stupidly expensive. So that really only left the dual-RCD board as an option.
As above, that is no longer the case - and a full RCBO board can cost less now than a dual-RCD board would have cost a few years ago.
OK, you're on another job (or on holiday), the other side of the country, and a customer phones up because "half the house" has gone off. They can't reset one of the RCDs even with all the MCBs off - so please explain how you will fault find "there and then" and leave the customer with everything but the faulty circuit working ?
Yes, some of the RCBOs have 2 pole switching - as do the SBS ones which are also just under ÂŁ20 inc VAT. You need to be careful exactly what's on offer - AIUI few of the single module devices have 2 pole sensing for the over-current, so they will switch the neutral, but not trip on neutral current (which shouldn't be a problem for most installs).
No, that would be reductio ad absurdum. But to a point, yes you are correct - it's why we don't (generally) put all the lights on one circuit, all the sockets on one circuit, etc.
A while ago I went to look at a fault in one of our lass's cottage - all the lights had tripped and she couldn't get them back on. There's an argument that at the time it was last re-wired, having all the lights (all 6 of them) on one fuse was acceptable. At some point, the bathroom (in a flat roofed extension) had been converted to a wet room, and as part of that they'd added an RCD to the lights. The fault was easy enough to guess - damp in an outside light.
These days I'd expect to see at least 2 light circuits in all but the smallest of properties - that's just one of those things where standard practices change over time.