"The Home Office Exchange Rate Policy applies the Bloomberg opening rate as the base exchange rate when taking payment in any currency other than sterling. All Home Office exchange rates are set at 4% above the Bloomberg opening rate. Rates will be reviewed on a fortnightly basis and only adjusted if they have varied by more than 1% away from the set rate i.e. below 3% or above 5% from the Bloomberg opening rate"
"If you have any further questions on the Home Office Exchange Rate Policy, including local rates of exchange and the associated refund policy, please contact us at the following e-mail address:
HOExchangeRates@HomeOffice.gsi.gov.uk"
Keep in mind that if you are trying to pay from the UK, you are likely being charged for two currency conversions. The first being the Home Office converting the GBP visa fee into the local currency and the second being your bank converting the local currency back into GBP (if applicable). No idea how or why euros may have become involved and I agree it's probably worth going to a Home Office or UKVI contact to confirm why the rates are that way and whether any additional charges were unintentionally added if you're concerned about the extra amount.
I'm just a guy on the Internet who immigrated to the UK. My opinions are based on my experience and interpretation of the immigration rules and should not be considered legal or immigration advice; your mileage may vary.