The big 4 expense policy is clear cut, so you don't have anything to worry about here. Just submit your expenses as normal, at £0.45 per mile, and it'll work out. If the expense is rejected, raise to the senior manager, partner or worst case HR. It's not at the accounts discretion. All that said, make sure you haven't misinterpreted. For example, you are expected to deduct your home to base cost before charging expenses, so if it costs you more than this for your current commute then you aren't eligible. Also different rules apply at manager grade and above, as at manager you begin to receive company car funding. Possibly the manager joined in that grade and doesn't know anything different.