Anyone here from a union or good with law?

[diversionsc]

Im currently in the process of typing up a letter which will be passed on to my personnel team at ASocitied DAiries.
It is concerning a dispute with a manager over a denial of holiday for the 9th of April (Easter Bank holiday monday) which was handed in on Sunday the 4th of March(5 weeks and 1 day notice, standard notice is 4 weeks i beleive).
Currently we (new world) are required to work 2 out of 5 bank holidays. On my behalf a fellow colleague approached the manager to "state" this information to which he replied "that he wouldn't be blackmailed by staff". This is one thing that has got me angry, she meerly stated to him the rules. He then said it is at managers discretion and that i hadn't given him enough notice, infact he said that i have only given him 2 weeks notice today. If you note that the holiday which i have applied for (my 20th birthday by the way) is on the 9th of April then as of today thats 3 weeks and 5 days away that he has infact contradicted himself.
He then approached me to offer a what i will consider false bargain stating that the store may only be open to 6pm on that monday and that (my shift being 5pm till 10pm) i could come in and work for 1 hour and then go off to enjoy my birthday. Knowing from previously working bank holidays that this is false stated what my collegue had said on my behalf, to which he replied in a rather annoyed manner "so you've been talking to *said collegue* then?" which i did not feel appropriate.
Final point, me and said collegue work this shift together, but she's on the old world contract currently and does not require to work bank holidays and wont be availible that day to cover. This said if her contract switches to new world then she to only needs to work 2 bank holidays thus in the best case scenario would meen out of the 5 bank holidays if we shared then it would leave one bank holiday not covered, or worse, if me and her both choose to do both the same days it leaves him with 3 days uncovered. I have stated if i can get this off then he can have me for the next 4 bank holidays. I feel its a fair deal as losing this holiday means that the careful planning of 4 of my friends for a day out go to waste.
Thank you in advance!