It is a pandemic. There is no reserve of uninfected, refreshed people.
I suspect a part of the reason for the call is to communicate that we are not in normal times and the idea of paying someone to make your problems go away doesn't work. Help or get out of the way, your payment will be a slightly less broken country when this is past.
The Government is providing 80% of your pay to anybody not able to work. Self-employed measures are being announced soon. Redundancies are limited in comparison.
Instead of or as well as doing that, they could pay people for volunteering to help the nhs. The other measures you mention are very costly in comparison.
Maybe they don't want the overhead of interviewing and taking on full employees expecting long-term employment at this time. I don't know if UK employment laws allows this (they are pretty strict) but perhaps they can issue a stipend for temporary volunteers who show up consistently.
These workers should fall under the same classification as other "gig economy" workers like Uber drivers. Whatever the relevant labor laws are in the UK, the government should be subject to them also.
Still worried about a slippery slope - free at the point of need. No relying on extra support. If we need it pay people to do the job.