The normal way to claim expenses when overseas is to record your actual travelling expenses (including flights, taxis, accommodation and meals).  You have to be disciplined and convert any foreign currency payments into Sterling for the actual transaction date.

However, it may well be worth using the HMRC benchmark rates for employees/directors who are required to travel abroad.  These benchmark rates give the daily rates for food and accommodation (not travel costs) that you can claim, without needing to keep receipts.

The benchmark rates, for various cities/countries, are available on the HMRC website and updated occasionally.