Can you issue a misconduct penalty without a formal disciplinary meeting...?

You should carry out a disciplinary even if its relatively short
In effect you have carried out the investigation

Thanks Keith, my thoughts exactly but i was advised by my previous HR manager that this is a waste of time as they have acknowledge their wrongdoing at therefore just issue letter with disciplinary penalty

IMHO any disciplinary letter should be handed over face to face by the manager, with full explanation and the possibility to ask questions.

Thanks Ray

your previous HR manager gave you bad advice. If there is a sanction applied, the proces should at minimum follow the ACAS code (as Keith says, the investigation stage can be deemed to have taken place, but the disciplinary hearing still needs to happen in order for the sanction to be considered and formally issued).

In short no, however it can be really quick and efficient in my opinion. Get the letter written with explanation, call the hearing and explain that there is no investigation due to the admittance discussed previously and hand over the letter. If at this stage he accepts then happy days, if he has questions you can answer them and if he disagrees you adjourn and go into investigation.

Thank you Ann, i thought this also

Thanks Alun , greatly appreciate

I would never do the letter before the meeting.

It fails to take into account any mitigation. ( even if you don’t Expect any)

It's self evident, I think that any formal disciplinary warning should be the outcome of a formal disciplinary hearing and in compliance with ACAS Code. Otherwise would be denying due process / natural justice.
It doesn't have to be a lot of rigmarole but IMHO it has to be done.
If it's not done right, then usually no great matter 'cos it's action short of dismissal but if dismissal follows as a result of it, then it may well be challenged and found to have been flawed, so best get it right.