I think the rules are that if the employer compensates you for the actual costs as incurred then fine, they can claim it as a cost of doing business and it's not income for you attracting extra tax. Or they can use the 'benchmark' rate, or they can use an agreed with HMRC bespoke rate. With these they just pay you the money. But you still have to keep receipts to show you are actually incurring expenses.
FWIW the benchmark meal allowance for an over-night trip is £25.
FWIW2 if the costs go over £25, or whatever bespoke rate you have, claim for the actual costs, employer should be happy with that.
https://www.gov.uk/hmrc-internal-manuals/employment-income-manual/eim30240 et al