What is your contractual situation? Do you have fixed part time or full time hours (either fixed in terms of exact times/days, or in terms of a number of guaranteed hours per week)? Or are you on a more casual arrangement, where you may get hours, but you may not?
If the former, then they can dismiss you without a reason because you don't yet have unfair dismissal rights, but you are entitled to be paid notice, plus if I were you I would kick up a fuss. So if after your holidays you are told that you don't have a job, I would suggest you write to HR straight away, telling them that you booked holiday, it was approved, then when you couldn't come in during your holiday you were dismissed without notice. Tell them what your deputy manager said on the phone, and that you believe you were dismissed because you were exercising your statutory right to annual leave. Say you would like HR to become involved to assist you to exercise your right of appeal against dismissal under McDonald's dismissal or grievance policy. Say that you want your job back, or at the very least you want to be paid your notice and any outstanding holiday pay.
If, on the other hand, you are a casual employee, then you may find that you just don't get any hours any more. In that case much the same. You should write to HR straight away, telling them that you booked holiday, it was approved, then when you couldn't come in during your holiday you have not been offered hours since. Tell them what your deputy manager said to you on the phone, and that you believe you are being disadvantaged on the ground that you exercised your statutory right to annual leave, and you would like to raise a personal grievance on that ground. Say that you want your hours back, or that alternatively you want damages for unlawful disadvantage.
Subtly different depending on your contract, but the action will be quite similar and tbh I would expect HR to come down quite hard on the Deputy Manager ... they don't like rogue managers who flout the law and make the company look bad.