RCD's do not stop you getting a shock when you touch some thing live, all they do is reduce the time you get the shock for, what we hope is they trip before we touch some thing live, which means in the main they protect from contaminated water passing current. But we can see where they help, we remove a socket after a trip and see the water in it, without it tripping it could have causes a fire.
But point is we do see where they have operated for good reason.
The SPD however we see nothing, same as filtered sockets, we have no idea if even some thing in the box. And why some are £30 and others £130 seems a mystery. Yes an RCBO varies in price from £12 to around £28 but £30 to £130 is a massive jump.
If the videos singing the praises of SPD showed the oscilloscope pattern before and after that would help, but pictures like this
View attachment 251193 with a claim that a SPD would have stopped it. I can not see how a SPD would have absorbed that.
However the question is what is the negative side to SPD, and the only negative is price, with RCD negative side is loss of power, so careful consideration is required. But with a SPD is designed to fail open circuit, so if not sure fit one.
However my fusebox CU came with a SPD fitted and no overload device for the SPD other than DNO fuse, latter boxes I see have a MCB to supply the SPD, which does mean the MCB has a resistance between SPD and supply, so it may not work as well.
With a RCD we talk about milliseconds must trip in 40 mS but with SPD we are looking as microseconds, not even sure if an oscilloscope would show it.
What we seem to be looking at is the use of LED lights and the risk of failure with a surge, since we can't other than fitting a SPD work out if a surge will cause the LED to fail, and unlike LED failing due to power cut where the emergency lights if fitted will work, a surge can take out the emergency lights as well, as also LED powered.
The other point is EV charging points with loss of PEN detection, the electronics which detect the loss of PEN can be damaged with a surge, so should be SPD on a feed to the EV charge point.
The same of cause with fire or smoke detection, electronic equipment so a surge could stop it working, so if that fails it would be dangerous, so with solar panels, fire alarms, and EV charging systems in the home with an EICR lack of SPD could be considered as a C2.
The point is many manufacturers as stipulating SPD before warranty is valid.
To my mind an emergency light likely the battery will act as a surge protection, but we don't know.
It seems for a single occupancy domestic installation SPD is not required, so can't see how you can code lack of SPD, however if a semi-detached house or even has a granny flat, would it be single occupancy?