Corporation Tax - Subsistence - What is "acceptable" by HMRC

A director is working in London and wants to ask HMRC for a higher rate of subsistence than what is the benchmark scale rate. What is the procedure for this. I explained to him: Overnight the benchmark subsistence rate is 15 hours (and ongoing at 8pm) £25 max amount of meal allowance. He says being a Director of a Company he has to put out a "certain status" and that London is "expensive" and you "cannot get a meal for £25 in London" ....He also mentioned taking out crew or Clients on the Exhibition jobs he is working on. I explained = Client Entertaining is not an allowable tax deductible, expense at HMRC. I explained £80 -£100 often, regular, for subsistence during working at HMRC would not be accepted by HMRC.

Hi Michael Beech,
There are no special 'rates' specific to London.
We would refer  you to the guidance here:
EIM30200 - Exemption for amounts which would otherwise be deductible: contents'
EIM30250 - Exemption for amounts which would otherwise be deductible: Bespoke agreements
Thank you.