Much of the effect of young men of working age getting killed off was mitigated by increased participation in the workforce by women.
Mirgration from the subcontinent had much more to do with the demand for cheap labour by manufacturers in the early 20th century, especially the textile industry, and by public services during the period of postwar rebuilding.
There was no significant population decline during or after WW2.
https://ourworldindata.org/grapher/population-of-england-millennium?time=1900..2016
Much of the effect of young men of working age getting killed off was mitigated by increased participation in the workforce by women.
Mirgration from the subcontinent had much more to do with the demand for cheap labour by manufacturers in the early 20th century, especially the textile industry, and by public services during the period of postwar rebuilding.