Many thanks for your patience. I fully agree that this is somewhat of a silly rule to have because as you said you are not actually leaving the premises or have any intention of taking the food without paying for it. However, this is a specific rule which they have put in place and as such you will have to adhere to it, especially if others do and the is no clear disregard of it by management or the general workforce, which has been ignored for a long time by the company.
Being on a break means actually being away from your normal workstation, it does not mean you have to be sitting down and relaxing and doing nothing. So whilst it may not be as much of an effective break as you may expect because you have to queue or deal with sorting out payment for your lunch, legally it is a break because you are not being asked to undertake your duties at tour normal workstation, like a desk or counter or till.
At this stage you are only being i vestigated, you are still presumed innocent until proven guilty. However, after the investigation they could decide to take this to a formal disciplinary hearing. At worst they should issue a warning, to warn you about future breaches of this policy and to remind you that you need to follow it.
If you wanted to challenge them then I suggest the best argument you have is linked to your diabetes, which can amount to a disability in law. In these circumstances the employer will have a duty to make what is known as reasonable adjustments to ensure you are not placed at a disadvantage because of it. So if the lack of proper break means your condition is affected and you are detrimentally affected health-wise, you can ask for dispensation from the rules to ensure that this does not happen. However, you will need to have a solid description of how it affects you, that it is more than just a trivial effect and that these changes are necessary in the circumstances.
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