Dean, yes it most definitely is happening in the UK.
I booked an apartment for the weekend in Spain and following advice from other users, selected price display in Euros which should have resulted in payment in EUR. I use a credit card with zero conversion fee and good exchange rates.
I was invoiced in pounds. I was charged GBP31 extra for the rental because of poor exchange rate and GBP16 extra for Airbnb's lovely 3% conversion fee. This, added to the very high Airbnb charge of GBP71, meant I paid over GBP100 to them. This is completely unacceptable and it may be illegal in European law because I am being charged more than a customer in Spain or another Eurozone country.
This is my first time using Airbnb and may be my last