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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-
Indeed, the + isn't meant to be taken literally, but there was probably no other way to put it on a slide that the majority would understand, so + was picked.
It was probably picked on purpose to be a distraction from the other parts of the message (which is mostly that the Government is shirking responsibility for the whole thing by putting the onus on the people to do what they think is 'common sense' rather than being told explicitly not to do a bunch of things that are going on all over the place and will only increase.)
Those Nightingale hospitals are going to be very busy in 14-21 days time.
Greenbank
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.