Hi @Sophie132,
Did you set up Pounds as your standard currency? Go to Profile - Edit Profile - then scroll down until the field where you enter the Currency, there you can select from a drop down menu.
However, I also a similar case recently - a think this is either a new feature or just a bug in the system - where I did send out a host offer to a British guest in EURO which is my currency - the host offer was shown in Pounds Sterling, after this the airbnb service fees were substracted in Euros and the pay-out amount was schown in Euro again. I checked the systems calculation manually and everything was fine, also the amount I finally received was in Euros.
Usually the guest sees all listing in his home currency (let's say Dollars), regardless which currency the host has defined for his listing. The host thereagainst is getting the booking requests etc. in his home currency (lets say Pounds), the guest pays in Dollars, thereafter the host gets the Credit in Pounds. It all works fine, just the host offers for guests with a different home currency are displayed in the guest's currency, but at the end it really doesn't matter how the offer is displayed - the system still translates correctly!