Can you state a source for this claim about Britain's population loss in the late 18th century? All the existing data I know is that Britain's population was rising rapidly during this time. 

The story about workers' living standards during the Industrial Revolution is complex, with different indicators moving in different directions, but a recent paper, Gallardo-Albarrán and de Jong (2020), put together various measures of real wages, working hours, health and income inequality for Britain in this time period.  They estimate that worker's welfare in 1850 was 22% higher than in 1760, and see two broad periods in this, from 1760 to 1800 where working hours and income inequality increased but so did life expectancy (by 5 years), and 1800 to 1850 where real wages rose, income inequality fell slightly, and working hours, after rising until 1830, declined. 

Life expectancy did plateau over 1825 to 1850 at the national level, and worsened in some urban areas, but the British population overall continued to grow. 

Note that the focus on real wages means that Gallardo-Albarrán and de Jong don't include welfare paid through the poor rates, which was a significant form of welfare in 18th century England & Wales. George Boyer, in an article for the eh.net website on the poor laws writes:

Real per capita expenditures more than doubled from 1748-50 to 1803, and remained at a high level until the Poor Law was amended in 1834 (see Table 1). Relief expenditures increased from 1.0% of GDP in 1748-50 to a peak of 2.7% of GDP in 1818-20 (Lindert 1998).

Boyer also describes a relative shift in aid towards recipients who were aged 20-59, rather than older, and to men. This may explain rising life expectancy without a rise in real wages from 1750 to 1800. 

Also, this is about all of Britain, while industrialisation started in northern and middle England: unskilled labouring wages in industrialising areas were higher. 

In short, industrialisation in Britain didn't come at a price in human lives: the population rose. Arguably, things like water, sewage and housing in the newly expanding cities could have been managed much better even given the technology of the time, but, as the Poor Laws indicate, policy makers at the time weren't indifferent to the plight of the poor. The picture of the British Industrial Revolution is complex, much more complex than politicised descriptions tend to make out.

Sources

Boyer, George. “English Poor Laws”. EH.Net Encyclopedia, edited by Robert Whaples. May 7, 2002. URL http://eh.net/encyclopedia/english-poor-laws/

Daniel Gallardo-Albarrán, Herman de Jong, Optimism or pessimism? A composite view on English living standards during the Industrial Revolution, European Review of Economic History, https://doi.org/10.1093/ereh/heaa002, copy at  https://academic.oup.com/ereh/advance-article/doi/10.1093/ereh/heaa002/5827898