The benefit in kind is usually assessed by reference to the cost to the employer of providing the benefit. In circumstances such as you describe, this would generally mean the cost of the food so provided.
There are now some situations where HMRC has managed to break the link between the taxable benefit and the cost to the employer (cars, cheap loans and accommodation have been around for ages but I am looking beyond those) and it is all about "salary sacrifice" or "optional remuneration arrangements".
This would be a scenario where the employer has said something like "we can give you the choice of either salary of £1,575 per year or food worth £7.50 per shift", in which case you get taxed on the higher of the two values.
Based on your description, if the provision of food has no alternative salary-based option, then someone has simply made a mistake and it should be rectified, to your advantage.
I suggest you query this with your employer initially, as HMRC is unlikely to be motivated to check an employer's calculations unless there's a lot of tax at stake and HMRC could make more money - which appears unlikely, in the scenario you describe.
It may be worth pointing out to the employer that it will also be to their advantage to reduce the taxable benefit down to cost, because the employer has a Class 1A NICs liability on employee benefits (payable only by the employer and nothing to do with individual employees) so a drop in taxable employee benefits means a drop in the employer's NI liability as well.
Just so you know, the statutory reference is ITEPA 2003 sections 203 & 204.
There is more guidance at HMRC's Employment Income Manual at EIM20020:
If the employer cannot / does not help, then you can dispute the benefit assessed on you personally by taking the matter up with HMRC directly and explaining your understanding of the position. Assuming you have work colleagues who are also affected, then if they want to reduce their own tax bills, then they will each have to raise the issue with their tax office, on an individual basis. (Clearly, it would make more sense if it were done centrally by the employer by revising all affected benefit Forms P11D as a single exercise, but each employee can deal with it at a personal level, if necessary.) I can imagine that some employers may be less than pleased if somebody went around telling his or her colleagues that their employer cannot do payroll properly, so discretion is advised.
Any further questions, get back in touch on this thread.
In the interim, I trust this will prove helpful.