Good morning! Always, always choose dirhams (AED)! If you select pounds at the payment terminal, that's called dynamic currency conversion (DCC) and it uses an awful exchange rate plus hidden fees that benefit the merchant. You can lose around 5% this way. Instead, insist on paying in AED and let your own UK bank handle the conversion - you'll get a much fairer rate. Same applies to ATM withdrawals - if the ATM offers to convert to GBP, decline it and choose AED. This guide for UK travelers explains it clearly:
https://www.poundsterlinglive.com/news/ ... hange-rate including a comparison table of payment methods and their exchange implications. Paying in local currency and using a no-fee card gives you the best value for your pounds!