Your company are treading dangerously as they probably have set a precedent by allowing you to wear the jeans you wear all this time, without questioning it. I employ staff and we supply shirts, aprons and hats, but my contract clearly states that employees must wear them at all times. I suspect that it should be written somewhere either in the company handbook or in your contract. I would take further advice on this one.
Also in case you did not realise, your company cannot take any disciplinary procedures against you unless you have 24 hours written notice even for a verbal warning, so they simply cannot suddenly call you into a meeting and then tell you that you have had your first warning. If they try this, then you will have extremely good grounds to win a tribunal in any future dismissal (if it gets that far!) Also they must give you the right to be accompanied by a representative of your choice in any disciplinary meetings.
Hope this helps, Sue