HMRC Benchmark Scale Rates vs Reimbursing from Receipts

Client (sole director of limited company) has dispensation for the company from HMRC which covers:

- Subsistence (including meals when undertaking business travel, supported by receipts) and

- HMRC Benchmark Scale Rates for Day Subsistence (£5 for one meal, 5 hours away; £10 for two meals; 10 hours away etc)

My client frequently travels on business and is usually away from his home for around 8-10 hours, and will usually purchase lunch while he is at the temporary site.

Does the company need to decide whether to use one method of expense reimbursement or the other, or are both allowed to be used depending on the cost in question?  For example, if the client spent £3.50 on lunch it would be more tax efficient for him to claim the £5 scale rate allowance from the company. If his lunch cost over £5, it would be more tax efficient for him to claim the exact cost of the lunch from the business.

I cannot see any reference to this in the dispensation notice, but would appreciate clarification on this point.