First up – I’m not a lawyer
But I do have a boss, and I do have staff
I work for a medium sized company – approx 250 employees, though we are part of a larger group.
As a manager I often have to attend various courses on the law as it applies to employees. We have well defined policies and processes for both capabailty (performance) management and disciplinary breaches
In terms of capability it is clear that it’s a multi stage process; informal meet to inform and dsicuss
If no improvement – formal meet, formal plan, possible verbal warning
if no improvement – potentially further (written) warning
Etc, etc. It’s actually quite hard to dismiss somebody who is co-operative but just ‘not quite up to it’
You make no mention of the size of your employer – if small they may not have the same requirement for policies and process. But ultimately they have to follow much the same. If they’ve made no effort to inform, help and coach they’re on rocky ground
Personally, I’d get on the front foot with them and lay it out – they can’t just issue a warning out of the blue. Probably it’ll scare the sh!tless. Buys you enough time to find a more enlightened employer