For a kick-off, slander is something that is said, libel is something that it written - so if it is anything, it is libellous.
However I'm not sure what you are after in terms of advice.
If you have been dismissed from your job instantaneously and without being paid notice period money, then your employer has decided you have committed gross disconduct. That's its opinion, and to challenge that and potentially get your job back, you need to go through the appeals procedure at work and if that fails decide if you wish to take the matter to an Employment Tribunal.
I wouldn't get tied up in the allegations that may be libellous - that would involve taking that person to court, involve hugely expensive lawyers and is normally reserved for 'stars' and 'celebrities' with oversized egos.
Your employer needs to make its case for your dismissal based on its investigation of ALL the evidence it has - not just one letter - and make its decision that (if necessary) can be shown to be fair under an ET.
I would try under the appeal to get the dismissal 'downgraded' to a Final Written Warning. Then keep your nose clean for the next 12 months until the warning period is lapsed.