I have worked for many large organisations sho themselves maintain certain rules regarding meal and subsistence expenses. They might, for example, rule that a person can claim a meal and one drink. They might even impose spending limits on the mean and/or drink.
I think, from an HMRC viewpoint, that the claim must be deemed reasonable.
Also, the expense is personal to the employee and not the firm because the employee is receiving a tax-free lump some of money when the claim is honoured. So it is for the employee to be able to reasonably justify the extent of the claim with HMRC, as well as the firm's policies. What is reasonable?
Not sure if this helps