The short-term mortality impacts of the lockdown-induced recession are estimated to be 4,500 fewer excess deaths (equivalent to 30,000 gained QALYs) occurring within a year of the lockdown;
these are expected to come from an estimated reduction in the number of fatalities due to a reduction in cardiovascular diseases, dementias and respiratory diseases
The report goes on to say this is because of "healthier lifestyles in the short term" - because of recession. There are a number of studies that link recession to (short term) reduced mortality - reduced obesity, reduced smoking, increased physical activity. Of course in the longer term there are negative affects, particularly around mental health and worsening health inequalities.
It's worth flagging that this report - all 188 pages of it! - is, explicitly, a modelling exercise and not a forecasting one. It's core "Covid Static Scenario" is a model based on a continuing level of 900 deaths a week from June 20 to March 21, for example.
In fact for the section discussed here, it says -
"This section estimates the wider morbidity and mortality impacts of the COVID-19-related ‘lockdown’ and the resulting recession. All of these estimates are based on arbitrary, purely illustrative, assumptions (see below for details), and therefore their reliability and robustness are considered to be very low. "