"£25 is an amount set by HMRC for daily rates when working away from your normal place of work in the UK and should be non-taxable. It equates to £7 for lunch and £18 for an evening meal. From what I understand, an amount higher than this would be liable to taxation."
I think you'll find that these are amounts that you don't have to produce receipts for (tho they may still ask for them)
Anything ABOVE these numbers may have to be justified - but I've never heard of anything reasonable being kicked back if you have receipts an it looks OK