You must have a face-to-face chat with you LM/TL.
False. Your Line Manager/Team Leader can ask you to stay a bit longer/come in earlier, and they can even ask why, but you are under no obligation to do as asked in that regard, additionally, such chats could easily descend into being perceptually intimidating, it also goes against the whole definition of the word 'Assumed'.
We don’t have enough staff in to cover on the team:
False. Teams do not operate in isolation, also, Fixed Start & End times on TPT scheduling are there to ensure we have adequate cover at all key points of the day, in order to provide the necessary standard of customer service.
My LM/TL told me that today is "Exceptional" so they can’t allow me to go.
Probably False. Unless something significant has happened that causes Business Continuity plans to be put into operation, then the chances of the reason being legitimately considered as 'Exceptiona'l as defined in Employee Deal are pretty much nil really.
Remember: Christmas, New Year, Eid, Easter, Summer Holidays, etc, etc, all happen every year and are all pre-mapped and known about in advance, before the demand maps are built, so they are NOT Exceptional (see paragraph 4) for the purposes of overriding Assumed Consent.