I think Van-Tam is working on the principle that a single vaccine is better than no vaccine at all, especially for those who are part of the "active" population, ie. those who may not be frontline but a necessary part of the functioning of society.
I haven't read anywhere yet that the vaccine producers have warned against the change in dosage. However, the UK MRHA said, "although optimal efficacy was achieved through two doses, both vaccines “offer considerable protection after a single dose, at least in the short term.”
Then there is the immediate problem of the logistics of the fill and finish part of the supply chain. Right now that part of the process is way behind the actual manufacture of the vaccine.
That's why I said earlier in the thread that IOMG should start making stats available about projected & actual immunisations per group. Without that data the public will assume the worst and that they are doing bugger all. If, for whatever reason, IOMG vaccine program grinds to a halt then it can only be because of supply chain issue outside of governments control. If they haven't already done so, IOMG should be drawing up plans to delay the 2nd jab as well.