Apparently, in the USA at least, death rates rise during periods of economic expansion and fall during economic downturns. I don’t know whether this holds in the UK as well. One possible reason for this is that when people feel well off they eat and drink more (and more unhealthily). Another is that people drive more, so there are more car accidents.
Yet another, according to a recent study, is that in good times nursing homes find it more difficult to hire care assistants because of the low wages.
Modelling future mortality rates is seriously difficult.