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It defintiely shouldn't be added - as the R rating in that makes do difference due to being swamped by the 10,000's of numbers of infections.
I don't know if it should just be multiplied though. I suspect not. I imagine they could have avoided the inevitable mockery from that slide by just annoucing that they had a model that took both into account.
christianSpaceman-
It's probably not multiplied either. Because a city with 20,000 cases is not twice as bad as one with 10,000 cases - they only differ by a couple of days of unmitigated growth. You wouldn't put double the effort into containing the virus in those places, you would put essentially the same effort in. What they really mean is:
COVID alert level = f(R, number of infections)
but people in the UK have incredibly poor maths education and a) don't know what a function is b) won't realise that a metric where 10,001 could mean anything from 10,000 + 1 to 9,997 + 4 is unbelievably daft.
frankenbike
Should that be multiplied rather than added?