There should be stipulations in their contract for dismissal during a probationary period.
A PIP can serve two purposes:
- Serve as a notice of "improve or you're out". This is a way to legally document the dismissal process for, typically, establish employees.
- Serve as a point of concern, and to genuinely attempt to improve the employee's performance otherwise they'll be sack.
If you're serving a PIP for the first point, to an employee in their probationary period, you may not need to. Review their contract, as typically UK employment contracts have a "we just need to give you X amount of notice" and you can sack them without reason.
If they don't have this clause in their contract and you sack them without reason, you open yourself up to a gross dismissal case which can be costly.
Check with your legal team/representative/HR for this clause in their contract, and act based on that.
Just seen your note about HR stating you need to put the on a PIP regardless.
Legally speaking you can fire them without reason in their probationary period, but if you have a policy in place that requires a PIP before firing an employee you should follow that. Don't attempt to superceed HR's advice. It won't end well.