You will have one major hurdle here and that is your length of service. If you have been continuously employed at your place of work for less than 2 years then your employment rights will unfortunately be somewhat limited. Most importantly, you will not be protected against unfair dismissal. This means that your employer can dismiss you for more or less any reason, and without following a fair procedure, as long as their decision is not based on discriminatory grounds (i.e. because of gender, race, religion, age, a disability, sexual orientation, etc.) or because you were trying to assert any of your statutory rights (e.g. requesting maternity/paternity leave, etc.).
So even if you have done nothing wrong, they can still dismiss you because you would simply not be able to challenge their decision. I know it appears unfair but it is the law.
Now, even if they can dismiss you, that does not mean that they will. So you still have a chance to save your job by trying to argue that the policy was never communicated to you officially, you were never told specifically of the rules of how to apply such a discount, you have used it (and I presume other have too) for some time with no issues, you were told by your supervisor that it is ok to use it and so on. These are all factors which will assist you and may try and sway their decision. However, remember that even if you have the best defence it may still not avoid a dismissal f that is the way they have decided this will go.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you