@DdraigGoch I don't think the timing point holds up (at least, not in all examples). For example, a traditional cookstove generates 1.5 to 4 tonnes of CO2/year. If you replace that with a modern stove the carbon saving to the planet starts the moment you switch. It costs about 20kg of CO2 to make and distribute the replacement stove, so you make that back almost instantly.
It is of course a counterfactual saving, i.e. less than would have been emitted otherwise. But that's exactly what we are trying to do, reduce CO2 emissions.
I haven't read Berners-Lee's book, so I'm just going off your quote. I agree with him on additionality (it has to be something that wouldn't have happened anyway). And I agree that it's important to have robust (not fraudulent) schemes. But I utterly reject his idea that it's important that we reduce carbon in an autarkical, short-termist way, each looking for our own personal savings, that delivery instantly, and only once everyone has done all of that should we then ask 'what are the cheapest and most effective ways, collectively, of reducing carbon emissions?'. I don't understand that way of thinking at all, other than as a sort of virtue ethics where what matters is the purity of motive not the end result. If you want to cut carbon as far and as fast as possible, you look directly to 'what are the most polluting things that can be removed/improved cheapest?' and divert funds there. That's the principle of carbon offsetting.
Trading so that the work gets done by whoever is best-placed to do it is what unlocks massive progress in other areas, and if it's massive progress we are after it seems silly to deny ourselves that powerful tool.
("you can't go and work as a doctor, or a teacher, or a mathematician, even though that's what you would be most efficient at, until you have grown your own individual radishes, stitched your own clothes from hand, built your home from your own toil, composted your own waste. Looking for mutually-beneficial swaps is just cheating, allowing some people to enjoy homes, clothes, food without the effort of making any real sacrifice, other than paying money for it. ").