Can I add RCBOs to a dual RCD board under a TN-S system?

I thought that AMD1 or even the vanilla 18th Ed would have required SPDs unless you did the risk assessment calculation to show they weren't needed, but the lack of them on an existing installation isn't a major safety worry. If needed they apply to all kinds circuits (or ones supplying new work at least), not just socket circuits. Adding them to an existing CU (especially one without any facility for connecting MCBs upstream of the RCCBs) is going to be tricky and likely not supported by the manufacturer. (didn't MK give up making CUs anyway?) A simpler option might be to add an SPD in the tails before both CUs - e.g. something like https://www.tlc-direct.co.uk/Products/WYREC2SPD.html (that's presuming a simple type 2 is all you need - if you have other incoming services, PV, lightning protection or an overhead supply the situation can get somewhat messier).

I agree - nothing wrong with smoke alarms sharing another well used circuit (usually lighting) - preferable in my book. In any event there would be no point in adding an RCBO downstream of one of the existing RCCBs - you'd just be increasing the number of possible points/causes of failure. If you did go for a separate RCBO you'd need it connected before the RCCBs but those simple dual RCD boards don't provide a bus-bar before the RCCBs so you're stuck without modifying the layout yourself which again probably isn't supported by the manufacturer's type-testing (although a few manufacturers do sell kits to do that kind of thing).

I'm more surprised your electrician hasn't spotted the exposed basic insulation by the Henley block. 

BTW - if your earthing facility is provided by a local DNO it's more like likely to be deemed to have 'PME conditions apply' - i.e.  you should treat it as if it were PME if that's the worst case, even if the supply is presented as TN-S, since DNO modifications in the street (existing or in the future) can effectively turn a TN-S supply into a TN-C-S PME one without you knowing.

   - Andy.

I thought that AMD1 or even the vanilla 18th Ed would have required SPDs unless you did the risk assessment calculation to show they weren't needed, but the lack of them on an existing installation isn't a major safety worry. If needed they apply to all kinds circuits (or ones supplying new work at least), not just socket circuits. Adding them to an existing CU (especially one without any facility for connecting MCBs upstream of the RCCBs) is going to be tricky and likely not supported by the manufacturer. (didn't MK give up making CUs anyway?) A simpler option might be to add an SPD in the tails before both CUs - e.g. something like https://www.tlc-direct.co.uk/Products/WYREC2SPD.html (that's presuming a simple type 2 is all you need - if you have other incoming services, PV, lightning protection or an overhead supply the situation can get somewhat messier).

I agree - nothing wrong with smoke alarms sharing another well used circuit (usually lighting) - preferable in my book. In any event there would be no point in adding an RCBO downstream of one of the existing RCCBs - you'd just be increasing the number of possible points/causes of failure. If you did go for a separate RCBO you'd need it connected before the RCCBs but those simple dual RCD boards don't provide a bus-bar before the RCCBs so you're stuck without modifying the layout yourself which again probably isn't supported by the manufacturer's type-testing (although a few manufacturers do sell kits to do that kind of thing).

I'm more surprised your electrician hasn't spotted the exposed basic insulation by the Henley block. 

BTW - if your earthing facility is provided by a local DNO it's more like likely to be deemed to have 'PME conditions apply' - i.e.  you should treat it as if it were PME if that's the worst case, even if the supply is presented as TN-S, since DNO modifications in the street (existing or in the future) can effectively turn a TN-S supply into a TN-C-S PME one without you knowing.

   - Andy.