Apart that is from the fact many are too sick to work, stuck on waiting lists for treatment in the NHS:
Maybe a few more foreign doctors, nurses, health professionals and auxiliary staff might help clear the backlog.
https://obr.uk/box/how-much-could-reducing-the-nhs-waiting-list-contribute-to-falling-inactivity-in-our-upside-scenario/#:~:text=We estimate that there were,-term sick inactive population).