As for amazon, paypal and many other retailers/payment processors: Yes, you can instruct them to bill your card in GBP and save some fees. Just make sure you choose to be billed in GBP, as amazon & paypal in particular like to try and be “helpful” anddo the conversion for you, unless you opt out.
My experience with Ryanair is that they set the currency that they charge you with based on your outgoing flight’s origin airport, and you cannot change that. Here a few examples of what I have actually experienced:
- Booking roundtrip UK > Germany > UK: Charged in GBP
- Booking roundtrip Germany > UK > Germany: Charged in EUR
- Booking single trip Germany > UK: Charged in EUR
- Booking single trip UK > Germany > Charged in GBP
Admittedly it’s a few years since I last tried, but I could not find a way to get ryanair to charge me in a currency that I wanted. In particular it didn’t seem to matter whether I used ryanair.de or .co.uk, whether I used a German or UK billing address, or whether I used a German (pre-Revolut times, I had an actual German credit card at that time) or UK card for payment.
I did at that time book two single tickets, as it ended up cheaper being billed at least one flight in EUR and I could not find any way in which I could convince Ryanair to bill my outgoing (UK > D) flight in EUR. Maybe (quite likely) others are cleverer than me (I would love to find out how for my next flight!), or maybe they changed their proccess since?