Stupid idea.
It makes the paperwork harder and there is no actual definition of a front line NHS role. The definitions vary from trust to trust for hospitals and GP surgeries. Are we exempting chemists as well?
Even where it's clear you're front line there is what about staff who have mixed roles, do they only pay a proportion of their tax, and what sort of proportion?
If you need a nurse to do a non-front line role do they keep their tax exemption, if so you won't get any volunteering to teach anymore unless you boost their band for that role to make up for it. And then what about someone coming from a non-front line role doing the same job?
It sounds nice but in practice it would be a nightmare and it would only be suggested by someone who either knows they won't need to do it, or someone who doesn't understand the NHS.