Hi,
There are two possible reasons for this -
1) Quotes always appear in your local currency; so if you are registered as living in USA, the rate will appear in US dollars, if you are in Australia then it will show as Australian dollars and so forth - it saves the guest the burden of doing the conversation of the different currencies themselves.
2)If the guest is based in the UK and registered as such on AIRbnb it would make sense to have the rates in £, they can easily rectify this by scrolling to the bottom of the listing page, left hand corner right underneath the language is a currency box, yours will show as GBP, there is a little drop down triangle in this box to choose their preferred currency.
Hope this is of some help.
Ama