When you buy a UK ETA (required for entry, at least from the US), they ask you what country you are located in, then charge you in that currency plus a conversion fee. There is no option to pay in GBP. The only way I could see to avoid the surcharge was to claim to be in the UK, which would be a problem, since it would mean you're there illegally.

The fee is trivial (about $1 on $22.50), but it annoyed me.